I hope I have the correct viewpoint on this but I seem to have Thumbgen working OK. However I always add 2 other files of my own, folder.jpg and MovieTitle.jpg (where MovieTitle is the actual name of the movie, eg The Help.jpg). Both these files are 120 x 180.
Then I run Thumbgen. The Thumbgen HowTo says to put a tick next to "Spare Movie Sheet" and set it as .png. I have deslected this. I will use the movie "The Help" as an example.
I finish up with a folder with 7 files:
This seems to work OK for me.
If necessary I get into the .xml sheet and and over write the with Crime/Drama This then fills the Genre heading on the movie sheet.
Sometimes I have to update the "Overview" with info I have copy pasted from either IMDB or Wikipedia.