Image Alchemy has the ability to convert between over 90 different raster image formats (including all colorspace and compression variations of each format). Color management features include gamma correction, undercolour removal, seven high quality dithers, colorspace conversion, and palette manipulation. It also includes a Postscript interpreter.
snapscreenshot takes a screenshot from a single Linux virtual console (tty) or from a group of ttys. It does not care if it is a plain text mode or framebuffer console; it only needs read-access to /dev/vcsa* -devices. The program outputs a tga file to stdout; use an image manipulation program to convert it to png or whatever you prefer.
SDLcam is an SDL GUI for the Philips Webcams that are supported by the Linux kernel. Images can saved after the filtering step as JPEG, PNG, or BMP. Most of the filters are optimized in assembler, and many use MMX, so they are very fast. It also supports FTP uploading with renaming. It has a very flexible config file that allows nearly all the options from the GUI to be preset.