Convert pnm to bw Online: pnm2bw
You can use Tom's Editor to easily convert images from
Portable aNy Map .PNM to Silicon Graphics RGB .BW.
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.
Silicon Graphics RGB is an image format with extension BW.