I was extremely lucky to get accepted in iStock on first try. I had sent in three shots all on food and the reply from the iStock inspector folks was that while they found the photos to be good technically - they are more keen to find out what else I can bring to iStock and whether I am creative in other genres or whether I can only just do food shots. So I sent back another shot of a travel and a creative studio shot and was accepted in the iStock fold. 

The thing I feel is for iStock the inspectors are looking at your images and asking themselves why they should bet on this potential contributor and whether the potential contributors can be an asset to iStock that in turns will offer something that can be commercially viable IN ADDITION to all their existing contributors.

Interesting enough, I send in my best shots at iStock to SS and thinking I will breeze through. Wrong, I was rejected twice and I will try again shortly. So maybe its not just a matter of sending in your best selling shots from other sites at iStock and expecting them to be accepted but trying to find out that extra little creative spark that will tell the image inspectors that they should bet on you and accept you into the fold.