Seems like its either having trouble loading the thumbnails, or generating new ones. Locate the folder containing the thumbnails and delete its contents, then open up the gallery again and let it generate them again.
No matter, it appears the thumbnail folder has been removed during a previous android update. It now seems to be incorporated in the cache file for the Gallery app.
To delete the cache, locate the gallery app, press and hold it until it gives you some options. Depending on your version of android, one of the options may be "App Info", or there may be a little "i" in a circle, a sort of "info" symbol, seen in my attachment image.
In the info screen, scroll down and locate the "storage" section, and click it to open. At the bottom of the storage section, there will be 2 options, clear data, and clear cache. Click clear cache.