The simplest raster image file format is probably netpbm (PBM, PGM, PPM), which is essentially an ASCII text file containing RGB triplets. However, it takes up a lot of space even if the entire image consists of pixels of the same color, because an RGB triplet has to be explicitly listed for every single pixel.
The BMP format is more complex than netpbm, but it seems that BMP can achieve much smaller file sizes by using a "color table" in its header to reduce repetition. BMP also has the advantage of portability. All major web browsers are able to render BMP images.
Are there other simple raster image formats that are reasonably portable? (i.e. can be rendered by most image viewing software).