how 2 do?
I would make you one and upload it for you, but you wouldn't see it.
Just go on Paint.NET > Image (Up top) > Resize Canvas > 75 x 75 > Eraser
When you use Eraser, you will see a checkered background as you Erase the canvas. This means it's transparent.
When you're done, just save it and upload it to your avatar.