Hello,
Did you upgrade from an earlier version > 6 > 7 or just from 6 > 7?
Also, how are you loading the images that are not rendering? Which Web part and how does the transformation look?
There are a few possible solutions 
here and 
here.
Have you tried clearing the server cache as well?  If the issue persists and is occurring for multiple images, you can try applying the latest 
hotfix to rule out any possibility of a bug causing the issue.