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.