A quick troubleshooting tip for Mac users:

If you are having problems with image icons in the Finder that look like generic jpeg icons instead of icon previews, try relaunching the Finder.

Select Force Quit from the Apple menu. Select Finder, then click the Relaunch button.

Or, delete the Finder preferences and then relaunch the Finder.

(~User/Library/Preferences/com.apple.finder.plist)