Hi everyone.
First post. I'm new to microstock and have been uploading to some of the smaller sites to get an idea, like 123RF, Bigstock and Canstock.
I've decided to apply to iStock and two images got rejected. I'm okay with that, I know I still have a lot to learn, but I don't understand the reason and I was hoping I could get some feedback. The one that was accepted is a simple shot on a white background and these two were rejected because of 'artifacting'.

Links to full size:
http://zumawebdesign.com/photos/app_sample_view-1.jpg
http://zumawebdesign.com/photos/app_sample_view.jpg

What is artifacting and where do I see the problem? How can I prevent it? I do see some purple fringing that I should have taken care off on the tree branches, but I'm puzzled by artifacting.
Thanks so much!