To put in an Avatar:
Click on Edit Avatar Settings in My Controls. (Or click that link.)
You can either upload one to the forum or link it from a website.
Example of a free webhost is http://www.imageshack.us/, you upload it there and they'll provide you a link to it. You have to NOTE that link.
Note: You may see your old avatar because of your browser cache. Press Ctrl+F5 in Internet Explorer to see the new uploaded avatar or clear your History.
Your avatar must be no bigger than 64 pixels by 64 pixels in size.
Uploaded avatars from your computer must be no larger than 35 KB.
The following file types are allowed: .gif .jpeg .jpg .png
To put in a signature:
Click on Edit Signature in My Controls.
Here, you can enter whatever you want into the text box.
If you want to put a picture into your sig (example is my sig), you need to have a webhost and upload it there, eg http://www.imageshack.us/. Note the link to the picture. Then click the "IMG" button above the text box in the Edit Signature page. You can enter your picture link there.
Click "Update my Signature" and you're done with your signature.
4 Lines of text
400 (width) x 80 (height) image with 1-2 lines of text
Max filesize: 60kb
It's annoying really. Comply with the rules or I'll have the "Signature Police" on you, a.k.a Moderators.
Any other suggestions, I'll note and update the first post periodically.