Sorry, but the dropbox images do not work.

edit: oops they are showing for me now.

okay - one of your horse shots is really nice, but 1) horses are boring stock, 2) there are lots of lighting issues. artefacts (in this case, small, white squares missing colour information)....secondly the shadows are very hard. direct sunlight should always be diffused, or if not diffused, the effect should be done very well.

the cheetah shot is also nice, but that one is quite flat. the colours are muted and the composition is a bit off. if you want to know what sells, do searches on major sites and sort by downloads to get an idea of the lighting and composition in best-selling images. however, don't copy someone's idea. build your own ideas, but use technical standards of accepted and well-selling images as your reference.

I didn't see the girl on the swing. I just loaded it, it's a nice shot, but again lighting. it's way too dark and flat. the focus also isn't that sharp.

Here is some "tough love"

http://dl.dropbox.com/u/16190936/wildanimalpark_247_wtm.jpg

Soft, I can't find sharpness in the images (at least not in the important parts. It's too hard to tell what the cat is doing, I have no idea what concept this image could illustrate. Also you might need a property release from the zoo.

http://dl.dropbox.com/u/16190936/la%20costa%20pools_325_wtm.jpg

It took me too long to understand what is going on here. The pose is awkward (especially the hand is in an awkward position), as is the expression (I can't tell if she's happy or terrified). Chromatic aberration in the highlights. The overall composition doesn't work

http://dl.dropbox.com/u/16190936/kylie%20hall_wtm.jpg

The text need to be removed in the "badge". An okay horse portrait, but nothing special, I wouldn't send this as my initial submission. Too tightly cropped top and bottom.

http://dl.dropbox.com/u/16190936/drivingmarathon_351_wtm.jpg

This one is too cluttered, it's (again) too hard to see what is happening. The crowd in the background distracts. Lighting is harsh.

http://dl.dropbox.com/u/16190936/faith%20hat%2012_wtm.jpg

This has a too flat and cool lighting. It also has lots of chromatic aberration. The bokeh looks bad in the feet. Top and bottom parts of the background are too cluttered. This in my opinion is the best photo of this batch.

http://dl.dropbox.com/u/16190936/fall_026_wtm.jpg

This is soft. Huge amounts of chromatic aberration, for example in the top right branches against the sky. The building is unappealing, I have no idea what concept this image could illustrate / lack of commercial value.

http://dl.dropbox.com/u/16190936/wildanimalpark_099_wtm.jpg

Too Noisy. The cat has some white stuff on his nose. Flat and dull lighting. The "platform" the cat is standing on says "zoo!". Property release needed from the zoo. The shape of the cat is too strange and unappealing. Would be much better if the cat looked in the camera.

http://dl.dropbox.com/u/16190936/foxes%20300%2014_wtm.jpg

Okay it's a fox A boring composition

Sorry if I sounded harsh, but it's important to hear these things from someone. I think your main weak points are: composition, post processing (the colors and tones need more OOOMPH!), chromatic aberration. You also need to focus to understanding that stock is not about taking pretty pictures, it's to shoot something that customers need. Either to shoot some subjects that are in need but there aren't too many of them already, or to shoot something in a different way or better. For example your fox image has to compete against these: http://www.shutterstock.com/cat.mhtml?lang=en&search_source=search_form&similar_photo_id=&searchterm=fox+animal&anyorall=all&search_group=photos&images_per_page=150 , if it's not better than the most of the images already there, it won't be sold. You need also always to think "in what kind of an article or advertisement could this image be used on?", If you don't know the answer it's often better to come up with some new ideas.

Instead of just looking through your photo folders on your computer, try taking pictures especially for the submission.

I had a couple of failures at iStockphoto, then I decided to ask a mother with two cute kids I knew. I asked them to jump up on their couch with a book (note, an old book with no text on the back for copyright purpose).
This is what the picture came out as. Not until I got home I regreted not seeing the black curtain in the background as a distraction, but I tried it anyways. The series of 7 pictures I took of this family has been accepted to all sites I have uploaded them to. And it was so easy! Not the best picture, but apparently good enough for stock.
Here is the picture: http://i51.tinypic.com/207aipk.jpg [nofollow]

I also decided to do some isolations, so I rolled out a white backdrop (white paper of anysort big enough will do) and took a picture of a guitar I had.
These were two of the shots (along with a winter image of Toronto skyline) I submitted to iStock and I got submitted. My previous attempts were with random crap I had in my photo folders from before I thought of joining stock photography, and they were rejected.

Good luck!

What Luis says is bang on. and you seem to have a great attitude, so keep trying and don't be discouraged. I posted in another thread about this that it took me five tries over six months to get accepted on iStock...and now four years later I'm a gold contributor and have learned more than I can express. keep at it and know that most critiques truly intend to be helpful, so take the good from the bad and really focus on learning to differentiate between nice photos and stock images. good luck.

Thanks again everybody.   I spent a little time yesterday figuring out some focus problems and working a little more with CNX2 on some of the other pictures to give them a little more pop.  Lots to learn.  This reminds me of golf -- no matter how much you've learned already there's a mountain of learning left.

Keep learning, you'll get there. My horse photos do well on istock, so don't be discouraged about submitting them. You have already gotten some good ideas about how to improve, so I can't really add any more there. It is much more difficult today to get in at IS than it was when I started, so I totally wish you the best of luck.