Two extremely easy and extremely small image converts.
InfranView: Converts anything to anything, jpgs, pngs, gifs, bmps, tff, etc. Also has any other features. Great for doing copy and paste conversions. I wish it had transparency option, but it doesn't. I use it all the time.
Giffy: Converts between bmp and gif and does transparency.
As I said, both are small and don't take long to download and can be very helpful. Btw, I use MSPaint to do most of my graphics too.