Hola Jane & John, welcome.
You asked about uploading an avatar that is obviously far too big for this forum in its current state. As you have realised you actually upload an avatar directly from your desktop but it needs to be the correct size.
Was the image you first chose animated, I wonder or just oversized ? You can resize animated ones (using Photobucket or another hosting site) then put the resized image back to your desktop & upload resized version from there to EOS. However, often it can still be oversized for avatar purposes though EOS are generous with 150 x 150 pixels, certain other forums only permit 90 x 90.
Just thought, you can go to this site http://resizepic.com/ & choose the maximum size EOS allows which is 150 x 150 pixels or 50.0 KB, whichever is smaller. So choose 150 x 150 or around that as not all are square then save it to your desktop. However, using this site it won't stay animated.
Good luck.