BMP is indeed one of the most basic graphic formats that are there, although there are several sub-formats as well. You have the option of using RLE compression (Run-Length Encoding) which can shave off a little size, but it's nowhere near as complex as the compression used for JPEG or GIF.
I wrote a PCX saver for PowerBASIC years ago, and didn't find it that difficult to implement the PCX RLE compression. Just looking at the GIF compression made my head explode, though. If you aren't too concerned about the compression thing, plain non-compressed BMP's are a very simple format, pretty much just a specified header with a raw data dump at the end. If you take a look at the BMP specifications at http://www.wotsit.org , you'll find it pretty straightforward.
I wrote a PCX saver for PowerBASIC years ago, and didn't find it that difficult to implement the PCX RLE compression. Just looking at the GIF compression made my head explode, though. If you aren't too concerned about the compression thing, plain non-compressed BMP's are a very simple format, pretty much just a specified header with a raw data dump at the end. If you take a look at the BMP specifications at http://www.wotsit.org , you'll find it pretty straightforward.