Okay, thanks everyone for the responses and the link.  Much appreciated (+1 Hearts all around).  I think I get it now.  I've done pieces both ways, but want be sure I'm keywording them properly.  Here are two examples from my own port that (I think) demonstrate the difference:


The shoes were shot against white, purposely high key to blow the whitespace out to pure white (255, 255, 255), but the there are some shadows left intentionally to add dimension.  This would be "On White".


The zebra was shot outdoors under natural light with a grassy background, removed, then and put on a pure white background (Also 255, 255, 255).  No BG shadows.   This would be "Isolated".
 

I'd used "isolated" in the keywords for each.  Is that wrong in the case of the shoes?  In the context of Microstock submissions, I had an understanding that the agencies' only qualifying criteria for accepting images that contained the keyword "isolated" was if the surrounding null space was pure white (255, 255, 255), which both of these have.  This was the source of my confusion regarding the difference.   I wasn't aware there was any other qualifying criteria, nor did I know you could isolate using other colours/backgrounds.     

I tend to just accept the rejection and move on unless the rejection reason is technical and something I can easily fix (even if I disagree with the reason).  Poor White Balance on a image that I had shot in RAW, would be one such example.   I will leave a note for the reviewer indicating I have reprocessed the original RAW file and specifically corrected the issue that resulted in the rejection.   

Another situation that happened just a few days ago when an agency rejected some photos for trademark infringement.  All other agencies accepted the images so I conducted my own research on the matter and found the rejecting reviewer to be in error in this case.  I resubmitted with a polite note regarding my findings, and asked that they reconsider their position.  They did, and the images were accepted.   Fortunately this type of rejection has been the exception, but people do make mistakes from time to time.

As a business, I think I would be remiss not to resubmit in these instances because I am walking away from potential income at no fault of the image.  At the same time however, continuing to try to push through images where the rejection reason is valid would be a bad business strategy - ultimately wasting my time, the reviewer's time, and clogging up the queue with images that have a higher chance of end customer dissatisfaction and thereby weakening my portfolio if they ever did get accepted.  The learning curve (that I am still working on) is knowing the difference.

Thanks for the suggestions!  For avoiding star trails, I use the 500 rule generously published by http://www.davemorrowphotography.com/p/tutorial-shooting-night-sky.html

For example, when using a full frame Camera @ 14mm, f2.8, for 30 seconds typically lands me at around ISO 3200 for proper exposure without star trails.  That's been pretty much the optimum for me with very little latitude.  Change any one of those factors I risk star trails, improper exposure, and/or undesirable ISO.

Attached are a few of the images that I am considering submitting that have used this technique.   They will likely need to be reprocessed for stock which is what led me to my original question before I put the time in.

Assuming it's an LCD monitor that uses cold-cathode fluorescent lamps (CCFLs) for back light it sounds to me like one of  tubes is failing or has failed (smaller monitors will only have one), or that the inverter is failing.  Complete failure will eventually leave the whole screen so dim that you will barely be able to see anything.  If you are (or know someone) handy with electronics,  depending on the model of monitor, you may be able to get parts to fix it, but if not, I would humbly suggest it not worth taking it to a shop and throwing more money at it, and replace the whole unit instead. 

Since aperture selection greatly impacts the Depth of Field and therefore the overall appearance of the resulting image, fast lenses also provide the photographer with more creative control than slower lenses - in addition to low light performance.

I'm not saying that everyone needs an arsenal of fast glass, but a 50mm 1.8 is so inexpensive and offers so much in that regard, I would recommend one to anyone.         

Dingles,  I see nothing wrong with wanting to stick with your D50 for now as you start out, it's a fine camera.  After 11 years in the photography business, I've just started contributing to Microstock this past year and have been uploading images from my back catalogue from older cameras including a D50 (which I still own) and  an even older D100.    I'm happy to report that images made with these cameras are being accepted and selling.

As for a fast lens, in 2003 I bought a used 50mm F1.8 lens for about $85 and it was a workhorse for me until I sold it 2013 on ebay for $75.  I'd say it's the best 10 bucks I ever spent, but in truth it paid for it self more times than I count.  I recommend it for your D50. 

Bad:

When you refer to your most recent rejections in terms of "batches".

Worse:

When you start to get image rejection notices from your "Self-Hosted" site.

I'm new to this crazy Microstock game having become a contributor only earlier this year.  I have no preformed opinion about any of the agencies, but do follow the boards here and elsewhere so am aware of the track record reputation of certain sites. 

So far with iStock, I'm cautiously optimistic.  Since changing their policies in the July time frame it has rapidly become my top earner, with the vast majority of it coming from PP sales.  My portfolio on iStock is slightly larger than my portfolio on Shutterstock, but my TS representation is less than my SS port, so I think port size is largely a wash. 

In September I had 50 downloads at IS with about 80% of them through PP.  I had 49 downloads at SS in September, so in terms of DL, they were neck and neck.  However, my IS revenue in September was double my SS revenue.

The other thing that I'm enjoying about iStock is the creative freedom.  The below photo was my most downloaded photo in September but was rejected at all other agencies usually stating "White Balance" issues as the reason.  This photo was taken with a 665nm Infrared converted camera so of course it has white balance "issues".

http://www.istockphoto.com/stock-photo-26753856-autumn-infrared-willow.php

My only real complaint is review times for non-exclusives (or me at least) are brutal -  usually taking 2 weeks or more.  I'm certain I missed a lot of content specific autumn/harvest related sales due to images just floating around in the queue.   

From Facebook's TOS:

�  For content that is covered by intellectual property rights, like photos and videos (IP content), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.
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So basically it could very well be her own fault.   She can sue, but I doubt she has much of a case if she's been spreading them all around the internet willy-nilly.