These websites are all nuts. I've been taking photos for a long time and I have accumulated a pretty impressive array of photos over the years. You can see my work at:
I was rejected because they claimed my photos were too "noisy." I'm shooting with Canon dSLRs (20D and now a 5D) which have some of the lowest noise levels in the industry. I know this because I'm particularly sensitive to noise and definitely try to avoid noise at all costs.
I can't be bothered...I'm making $500 a month steady income on photos as a part time gig while I'm in law school, and for someone to sit there and tell me my submitted photos were noisy (they were most definitely NOT) shows just how illogical they are.
If others can get accepted, great, but based on my experience, I have no idea how.
Can't determine if they are noisy or not without a 100% crop. Sure it could look great at 800x600 or whatever, but if it's not noise free at the full 4 megapixel limit, or whatever size you uploaded, then yea its going to get rejected.
Those photos all look great and would likely be accepted if they had no noise at 100%. (You can always shrink them down to 4MP and then upload them, or do a slight blur in photoshop)
Got my new camera the other day (400D), so I can finally start to submit photos instead of just all illustrations. Here are a few of my first ones accepted to Shutterstock: