The composition is good, but the artefacts are unfortunately very very pronounced in the blue sky, canyon background, and on tree barks. The pine tree on the left has very bad red and green fringes as well.

What kind of camera did you use? Did you sharpen it? Did you turn off the in-camera sharpening? If you shot RAW, you may want to reprocess the photo from the begining.  You also need to reduce the constrast a bit. Good luck!

I feel the worst problem is over sharpening.  iStock REALLY doesn't like over processed files so keep edits to a minimum, especially with things that seriously alter the entire image like sharpening and noise reduction.

One tip, if you really want to get a borderline picture online is to reduce the size of it.  This also reduces the size of any potential problems making a borderline line image acceptable.

Thanks everyone for replying. I think I understand where artifacting is coming from, but I'm still not a 100% sure what it looks like.
Is it this sort of white line along the bark artifacting?
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The sharp line where the rocks meet the sky as well?
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I do have the original file of both of them, but unfortunately I didn't shoot RAW. I have turned off the in camera sharpening though and I'll try to reprocess as soon as I'm clear on what exactly I'm trying to avoid.

Again, thanks everyone for the help, I really appreciate it!

Yes, as everyone already said, you probably did over sharpen which causes the outline. but the rest of the image is soft, or over process to lose detail.
Are you using in camera noise reduction ? If so, it could do with less . As for the artifacts in the sky, you can correct that by using layers to blur for that portion.
There is an overall lack of definition, which might be due to your camera or/and lense. Hard to tell . Are you shooting at wide open aperture? Is that a zoom you are using? If yes, try to experiment with find the critical aperture (sweet spot) by stopping down until you get the sharpest part of the lense. Also, if your camera is noise reducing, try not to do that, and use a software to do all this post processing.
hope this helps. There's too little information to be truly able to say what the problem is.
Can you furnish us with more info. Camera, MP, lense, fstop used, focal lengths, sharpen+ , noise reduction +,etc.
Whatever, as already said, try shooting only in RAW then use a software for post processing. You'll have more control and better results.

Thanks so much everyone. I have a Canon XSi and these were taken with the Canon 10-22 in large jpeg mode. In camera noise reduction is turned off, highlight tone is disabled, auto lighting optimizer disabled, but for picture style I had sharpness on 3, contrast +2, saturation +1 and colour tone +1. I've since disabled the sharpening too and will try to shoot RAW from now on. I'm using a Lightroom trial at the moment, but it seems to have a lot of problems with importing the XSi RAW, so I haven't been using it enough.

Thanks epantha for the crop, that helps a lot!  
I will try to reprocess and watch my settings a bit closer from now on. And I think I have to move my computer to a darker place in the house I have a heck of a time making it out, but I see what you mean. Thank you!

I think I got it. Here's the reprocessed file:

http://zumawebdesign.com/photos/stock4-1.jpg [nofollow]

How do you guys feel about that one? Looks much smoother to me.
Is there a point resubmitting this to iStock when I apply again to show I can learn, or should I not bother and submit new images?

I'm so glad I found this forum, you guys have been incredibly helpful!

generally, it's best not to do any sharpening, NR,etc.. in camera. reason because the processing is irreversible. i always prefer to shoot RAW and use photoshop to do all adjusting. also shoot at the highest resolution. and if it needs sharpening, do it in RAW. many times you don't even need sharpening, just downsizing alone can produce good results without any further sharpening.
ideally you should shoot at the sharpest aperture. you can find this in lense reviews , google it . many reviews will tell you which aperture perform the best .
then instead of doing general sharpening on one image, i prefer to use layers so the sharpening is specific in a certain area . layers are the best way to go, as you can use this to correct specific problems, one at the time. all without ruining the entire image.
hope this helps.

The sky is better, less grainy. I can still see CA on the left along the edge of the dark sandstone. The edges of the sandstone silhouettes are still over sharpened. The middle ground looked over-filtered through too much noise reduction, especially around the white tree branch (or rock ) on the right. I doubt if it will pass if you submit the full size. However, you can try to reduce the size as Whatalife suggests.

Your camera is not the problem. I know some IS diamond contributors are using the same camera or even earlier versions of the Rebel series.

Now that you know about artifacting you can work on your photography technique. 
The tree at the Grand Canyon:  Landscapes are best shot during the "Magic Hours" that is the time just before and just after sunrise and sunset.  The tree looks to have been shot closer to mid day.  Don't cut the top of the tree off next time.

John Ford Point at Monument Valley:  You have captured very nice clouds but they would stand out better in the evening.  Try to clone out the top of the RV on the right.  Next time either move the horizon down a little more, this will show more of the sky, or move the horizon way up and have something, a cow skull or a canteen with water coming out of it, big in the forground.

Mike Norton

This was a great discussion. I'm new here and learned a whole lot about the artifacting and the sharpening process when submitting. Thanks to you all. And congrats on your acceptances1