Hi John.
The nutshell answer is BMP is a direct representation of everything needed to create the image on screen while GIF, JPG, and PNG are compression methods to consolidate the information.
In terms I know you can directly relate to, a BMP chainsaw will stay sitting right on the bench while you pull the starter cord 'cause it ain't got no compression. The other methods can offer so much compression even <i>you</i> couldn't drop-start them without a few release valves.
JPEG, in particular, is best suited to "natural" images (like photos) because in achieving the high amounts of compression it throws out a lot of fine detail, averaging stuff together. Look closely at an enlarged portion of an image and note the checkerboard patterns. And I'm not talking about the square pixels themselves, rather the way they're arranged.
In my example image, the top is an enlargement of a PNG image and the bottom is of the same image JPEG compressed.
The averaged "lossiness" of JPEG isn't too noticeable in "real" images but it's "outstanding" in such as my example. PNG isn't very good at stuff like the pic of Danny because in that type of application it can't offer very much savings. Here's some relative sizes:<pre> 921654 danny.bmp<br> 642559 danny.png<br> 80868 danny.jpg</pre>It should be noted that JPEG isn't the best for images like the one I posted earlier (and which is enlarged in this current attachment); the PNG version is 7833 bytes, the BMP version is 50468 bytes, and the JPEG version is 55407, so not only did it wind up uglier, but larger.
I hate to spoil it for you, but MPEG is very similar to JPEG and now you'll notice the checkerboard patterns along hard lines on satellite TV and DVD, which are so encoded. It's a curse knowing this crap; I can't enjoy anything. Well, not really; I don't want to know how beer is made.
By the way, it looks like your photo had already been a JPEG before because the tell-tale checkerboards are present.
Glen