Hi GP
I haven't done this myself on this forum, but I suspect the answer is as follows:
First, you need to either build or find the animated GIF file that you want to add to your profile as your avatar (which is what forums like this one call the icons that appear next to your name). If you don't already have one in mind, see the sources at the end of this post.
Once you've found one you like, you need to use your browser to download it to a file on your computer. Depending on which browser you are using, this might be done in different ways, but all of them will be something like: "right click on the item, then choose 'save image to file'".
-- At this point, you have a file on your computer with a filename that ends in .gif
Now, log into sex141, and go to the "Member's CP" menu (near the right side of the page, near the top), and choose "Edit Profile". This will bring up a page where you can edit all sorts of info about yourself for the forum.
Near the bottom is section called "Avatar". Click the "Choose File" button, and select the file you saved to your computer. Once satisfied, push the "Submit" button at the bottom of the "Edit Profile" page. This will update your profile with the new avatar.
In theory, that's all there is too it
There are all kinds of online sources for these animated gif avatars, such as:
http://avatargifs.tumblr.com
http://avatars.jurko.net/browse.php?sort=best
http://www.animated-gifs.eu/avatars-100x100-girls-sexy/001.htm
http://www.avatarsdb.com/animated/
http://www.animated-gifs.eu/avatars.htm
http://gif-avatars.com
Gherk