You can request anyone's avatar with
curl like this on MacOS X:
curl "http://gravatar.com/avatar/$(md5 -q -s firstname.lastname@example.org)?s=300" > avatar.jpeg
Or like this on Linux:
curl "http://gravatar.com/avatar/$(echo -n "email@example.com" | md5sum - | cut -d' ' -f1)?s=300" > avatar.jpeg
firstname.lastname@example.org with your (or anyone else's) email address, and
300 with the pixel size (up to 2048). For example,
s=300 will get you a 300x300 pixel image.
If you don't have access to a Unix command line, or would like to figure out the URL to enter directly, you'll need to generate the MD5 checksum of the email address. You can do that via various online tools, like this one.
Once you have the MD5 checksum, just plug it into the URL. For example, mine is
51e61f099c7b1ac0588ecda69193f75c, so my Gravatar URL is
http://gravatar.com/avatar/51e61f099c7b1ac0588ecda69193f75c?s=300 for a 300x300 JPEG.