As far as I experienced in a short time , I believe that stock images must have these three properties;
1 - Quality ..
I mean technically it must be perfect. Focusing and lighting must be correct.
If isolated than background must be properly removed and edges must be clean and natural looking.
Image must have a concept. Ask yourself what is the message you want your picture to deliver to the viewers
3 - Keywords...
Choose your keywords which describes your image best.
And finally, always learn from refusals and criticise your images before uploading them.
Thanks for reading