Convert pnm to bkg Online: pnm2bkg
You can use Tom's Editor to easily convert images from
Portable aNy Map .PNM to Movie Maker Background
Tom's Editor is a free, online image manipulation and converter program.
Go to Tom's Editor
PNM images are uncompressed 24 bit RGB images. Can cannot store Alpha channel. PNM images come in 2 types: binary and text. Especially popular on Linux.
Movie Maker Background
is an image format with extension BKG.