FYI, appending .jpg to a bitmap isn't always going to work. In newer OS'es (MS Paint XP, Vista, and Win7), it will save as a JPEG. In older OS'es (Win95/98/ME/maybe 2K) Paint will just save it as a bitmap with a .jpg extension (unusable). Make sure you're actually saving it as a JPEG, not just appending ".jpg" to the bitmap's filename.
Anyways, JPEG has a much smaller filesize. You won't notice a difference if you use good quality. 100% quality on a JPEG gives you pretty much the perfect BMP and it is still much smaller than BMP. JPEG allows for different levels of compression, the lower the percent, the poorer the quality of the image, but also the smaller the file size. If you don't need photographic quality, you can save JPEG's at half or even 1/4th quality and people can still at least read the text or get the general image with much less filesize than a full quality one.
Re: Saving pictures in jpg 
Linear Mode
