I'm not sure, yet. Nor do I know why the one that I added shows up differently in the thread. I just checked and the file sizes range from 29 KB to 613 KB. The second one is a .BMP. The others are .JPG's.
Do you know of a thread that has one or more thumbnails that takes more than 2 or 3 seconds to load? If so, I'll go and see whether it's slow for me, too.