Shutterstock probably has the most difficult review process to get accepted of any of the microstock websites. You have to submit 10 photos for initial review, of which 7 must be accepted (you must wait 30 days to try again if you aren't accepted). Shutterstock has a great community, the best forums, and excellent downloads (earnings). so don't give up.
1) Microstock = Commercial, so when images are reviewed the reviewers are looking for saleable images. All 10 images submitted for review should have a clear commercial value.
2) More Details - Make sure all 10 photos you submit for acceptance are technically perfect.
3) Isolations - Images that are isolated on a white background tend to sell better than non-isolated images. Because of this, if you include isolations in your batch of 10 photos for approval your chances will be improved.
4) if you are good in illustration, you can use illustration save in JPG for submit, more chances.