Canon camera's can store the images in RAW format, which is --I believe-- proprietary.
Canon includes a CD with application programs for Windoze, which can process these files, but that doesn't help here.
There exists an image converter which converts from .crw to .ppm. It is called dcraw
You can download the source from that site. For Debian users, the binary exists in the dcraw package. Another package, gimp-dcraw provides a plugin for The Gimp.
Both work like a charm.
I haven't tried but it seems dcraw can handle many more formats besides Canon.