Thumbnails generation failing

I´m trialing SmartFTP and i´m quite happy with it. It does what i was looking for (generate http links for files, image previews). However i´m having trouble to make it display bigger images. I´ve followed the instructions in the tutorial video (thumbnail view demonstration) and changed the sizes of the background transfers. But the images still don´t display even after a system restart.
What am i doing wrong? I´m willing to buy the program, but this is one of the features i need more and i can´t get it to work.

Maybe the max file size is not big enough?

Nope. The max file size is big enough. There´s something strange going on: i have two images the same size (1024x800px), both jpg, one with 158Kb, the other with 433Kb. The first  (158Kb) shows a small thumbnail; the other (433Kb) show a big thumbnail.