Size
Width and height of future thumbnails. Thumbnails are scaled using "No Border" mode. This means that if you need square thumbnails and source movies are wider, they will be cropped in width. During tests images up to 8000 x 8000 pixels were successfully generated. Actual maximum image size depend on your computer configuration
Back Color
Some movies have transparent areas usually displayed using pre-defined back color. If you prepare thumbnails for a web site, you may wish to set this to some key color of your web site color palette
JPEG Quality
Thumbnail image compression in per cent. Higher values give better image quality at the cost of file size