Dear Jamie,
You can use an Avatar up to 20k. However pixel requirement is 50 x 50. You just have to resize the picture as long as it is under 20k.
Method:
1) Save picture under 20k
2) Take your picture put it in your favourite graphics program and adjust the dimensions so that the picture that you want to use fits the 50 x 50 requirement.
If you do not have a graphics program just use "Paint" in your windows accessories folder.
Then go to image; attributes; there you can change the width and height so that it is a maximum of 50 x 50.
Cheers,
Sunstone.