not really - the size of images is important in the running of the wiki chunk. Say the bit on weapons - there is _alot_ of images going in and if they remain as BMP's it gonna take years to load. In the lower resolution images PNG might be the best bet but when an image is that small it might be just as easy to use highly compressed Jpegs instead.
Jpeg is lossy compression, that means you will loose quality no matter what.
Jpeg is good for photographs.
Gif is the only one that can be animated, it can only be in 256 colors so it suits the simple graphics better. Gif can also have simple 1bit transparency.
Png is lossless so it results in larger file sizes. It can however compress in either 256 colors like gif or millions like jpeg, it also supports smooth 8bit transparency. However Internet explorer doesn't support that, but IMHO screw broken browsers.
The best for the pixel art is Gif, since those are small and won't loose much quality if you reduce em to 256 colors (custom palette) also, the 1 bit transparency doesn't do much when the edges are rough such as in pixel art.
Finally, don't even think of compressing with MsPaint.