There is no standard method of storing images in files. Instead, there are a vast array of file formats. Some of these are widely used (e.g. GIF, JPEG) and serve as exchange formats. Others are adapted to the needs of particular applications or particular operating systems. Many commonly-used formats are described in Murray and vanRyper.
The most useful package for translating between image formats is Pbmplus. This package contains Unix programs to translate between many formats and a simple generic format (pbm/ppm/pnm). Two of these programs can be piped together to translate between any two of the supported formats. See also Joseph Walker's page on graphics viewers, editors, and related utilities.