@bathsheba,
ok, once you've logged in to gravatar:
My Account --> Add an Image
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select an image from your PC
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click
Browse, select the image, then click
Next
now here's the tricky part:
Crop and Finish
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grab a corner of the white dotted line until you see a double arrow, and drag until you are happy with the two previews on the right.
do the same with the opposite corner.
you can also grab a side to stretch the image.
when you're happy with the previews, click
Crop and Finish!
next -- choose a rating for your image (G,PG, R,X).
next -- choose the image and click
Confirm
it may take time for your avatar to appear on A2k, so be patient...