Should be able to do anything you want once you purchase it. Might want to check their FAQ or shoot them an email and see if there's some sort of licensing restrictions but I wouldn't think there would be. However, even with licensing restrictions you'd be able to resize it though I'm not sure about croping it since that may bee seen as "modifying" the image.
Are there rules against modifying the image? I doubt there is. They are royalty free, so the photographer couldn't go after you, it would just be whatever rules the stock photo site has.
you can edit the pictures but can't give it away, resell or do any such things that violates their copyright policy.