You can open the picture you want in Paint then with click Image in the tool bar and hit sketch/skew.
resize the pic by putting 70% or 50% in the first two boxes. Hit enter. Then save it under a new name.
This will get pics down to the allowed size for use as an avatar.
Rigtalk is cool because this board has the biggest sized avatars allowed that I've ever seen on a guitar forum. Usually you are only allowed 12kb and 100x100.