The netpbm tools (a supported version of the earlier pbmplus, etc, tools) are a set of command line image conversion and manipulation programs that center around four very basic image types.

PBM: portable bit maps -- black and white, one bit per pixel

PGM: portable gray maps -- black and white, typically 8 bits per pixel but other depths supported

PPM: portable pix maps -- RGB images, typically 8 bits per pixel but other depths supported

PAM: portable arbitrary maps -- the newest format which can include any of the above, but has the flexibility to expand to other color models and channel counts.

There is also "pnm" with isn't a format, but a designation for a multiformat program (portable any map).

The pbm tools have been around seemingly forever on Unix systems, and are now available for Mac and Windows systems. The philosophy of these is that of Unix: many small tools that each do a single thing well, and all of which interoperate well.

http://netpbm.tribe.net