Do you have a copy of Microsoft Photo Editor? (I have it on my computer & I didn't buy it - suspect it came with some other Microsoft tools...) If you open the photo there, use the image... resize... option. If you set the units to pixels, then set the larger dimension to 100 (to meet the 100x100 requirements for the avatar), the other dimension will be resized automatically. I'd recommend saving the resized photo under a different name than your original, just in case you want (or need) to get back to the original photo.

--- Denise