I just put a bunch of my illustrations up on shutterstock. My initial impression is that I like the contributer interface better than istock. It’s easier to upload, you get approval much faster, and they don’t penalize you if it takes a few tries to get an image into acceptable form. Also, they accept editorial illustrations, unlike istock. That means I can use them to sell limited rights to any of my political cartoons that have caricatures of famous people in them. Their payment threshold of $35 with PayPal is also lower than istock’s. I’m not sure how their royalty rates compare yet.
Check it out and let me know what you think.