Hi, if 20/20 won't work for you, try downloading Irfan viewer. It's free and takes about 10 seconds.
All you have to do is open the image using Irfan, then Resize/Resample the image. It allows you to put the dimensions in. 125x125 is the avatar limit, I believe. Also, if you're working with a .bmp, make sure it saves as a .jpg so that the file is smaller. Good luck!