Among the oldest image formats currently supported is Microsoft's Bitmap (bmp). The bitmap is very simple: it is simply the basic grid of colored dots with no compression. Because bitmaps are not compressed, they can get very large very quickly. For instance, an image with a resolution of 800 X 600 has 480,000 pixels. If it has 8 bits per pixel (256 colors possible), the file will be 480KB. That's one byte for every pixel. Suppose you want a 24-bit image, though? Your image shoots up to 1.44 MB. 800 X 600 is pretty small. If you're wanting a good-sized battle mat, say 24" X 36" at 200 dpi (dots per inch), you're talking about a resolution of 4800 X 7200. That's over 34 million pixels, and at 24 bits per pixel, you've got an image that requires more than 103 MB of memory just to load, to say nothing of emailing it to a friend for critiques.
The advantage to using the bitmap format is that it requires very little processing to manipulate an image. The computer is not forced to decode a compression scheme, so changes are very quick. That can be an advantage when dealing with complicated computations, although the processor speeds of most modern computers make the time savings minute.
Bitmaps are best used for small objects that are being prepared for extensive processing. The format does not support transparency, though, so its use is limited. Also, many internet browsers cannot natively display a bitmap. All in all, the bitmap format is largely obsolete, although it is still the default file format for MS Paint for some reason.