Corrupt tumbnames, mismatch between server filename, and dB filename
If you edit an image, replacing it (2 times or more) with another image having the same file name, the thumb filename in the database will be different than the filename on the server.
This happens frequently if you use the same filename, like "fun.jpg", "new.jpg", etc.
To reporduce it:
1. Uplaod an image called new.jpg
2. Edit, and replace with a 2nd image named new.jpg
3. Edit, and replace with a 3rd image named new.jpg
Now browse the category, the thumb image will not appear, but if you click on the thumb title, the image is there.
Check the database for the image id, and you see the filename is appended with a random number, say:
new_704636.jpg, and the thumbname is appended a different number:
matching image edit #2, and missing the "_thumb" postfix.