Are you referring to "attachment thumbnails" or images via [IMG] bbcode?
If "attachment thumbnails"..
I think
postbit_attachmentthumbnail is the template you are seeking.
Have you considered rebuilding your attachment thumbnails?
Rebuild Thumbnails - this will rebuild the thumbnails whenever you change thumbnail dimensions.
If [IMG] bbcode..
I believe you will find that in the class_bbcode.php file.