Convert otb to pnm Online: otb2pnm
You can use Tom's Editor to easily convert images from
On-the-air Bitmap .OTB to Portable aNy Map .PNM.
Tom's Editor is a free, online image manipulation and converter program.
Go to Tom's Editor
On-the-air Bitmap is an image format with extension OTB.
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.