My tips for getting past that initial 7/10 hurdle with SS:
Forget about picking images you think will be good sellers, rather go for photos that are technically good. I'm not sure if they are stricter with the first ten - I got 10/10 accepted and later a lot of photos rejected, but you will have lots of opportunity to upload good sellers later. Look for something with natural lighting because it lessens the possibility of problems with white balance. Even if you are satisfied with lighting of the photo, make sure the histogram also tells you its good. I sometimes get the impression some reviewers look at the histogram rather than the photo itself. Make sure the light source was behind you, reviewers tend to find more noise in shadows or back lit images. Don't sharpen photos at all. With SS I never make any adjustments in Photoshop, except for levels and selective noise reduction. Look at every part of the photo at 100% view, and if anything looks just remotely like there could be noise or jpeg artifacts, enlarge that part to 200% and get rid of the problem, or don't submit.