Here’s a quick fix for the Finder not displaying a thumbnail of an image for it’s icon.
Drop into the Terminal and type:

qlmanage -r cache

This resets the QuickLook cache. As QuickLook is responsible for extracting the thumbnails for icon previews, this will also force any supported file types to have their icons refreshed – depending on the speed of your Mac, it may take a few minutes for them to all update.