Welcome to the forum Matt,
First a couple of system settings to check.
Make sure "Show thumbnails instead of icons" under Start, R-Click (My) Computer, Properties, Advanced System settings, Advanced Tab, Performance, Settings Button, Visual Effects Tab is ticked and that "Always show icons, never thumbnails" under Windows Explorer (any open folder), Tools (in the "Menu Bar"), Folder Options, View Tab is unticked.
Also. Delete your Thumbs cache files with the Disk Cleanup Utility (Press Start, (My) Computer, Right click C: Drive (your Main or System Drive), Properties, Click on the Disk Clean Up button). Thumbnails is one of the options you can choose to clean up.
Then delete your (hidden) IconCache.db which you'll find in C:\Users\Your User Name\AppData\Local (This last one is just for fun and to give you a clean slate).
Finally re-start.
Could you then tell us which thumbnail images are not displaying. Is it just image files? Maybe even a couple of screenshots.