Userpics/avatars
To edit your userpic/avatar, click on your userpic in the user sidebar (people without a userpic will see a default image):
To change your userpic, scroll down to the userpic area and click on "upload image"
Once you've chosen an image from your computer, make sure you click the "upload" button:
You'll know your userpic is changed when you see the new picture in the userpic spot:
Guidelines about userpics:
They must be square, and a maximum size of 100x100 pixels
They can be under 100x100 pixels (as long as they're still square)
If the image is not square or is larger than 100x100, it will be resized to fit
They can be .jpg, .jpeg, .png or .gif
Transparency and animation is supported, however stuff like crazy flashing animations aren't acceptable
Keep the filesize reasonable: under 50kb. If your userpic is too large you will be asked to change it, or an admin will change it themselves if the file is very oversized.
Keep in mind that your userpic can be seen by ANYONE browsing the site, so all userpics must be worksafe. No "adult" userpics.