If you want to make an avatar for yourself, all you have to do is create an image (jpg or gif) that is no more than 90x90 pixels and 6 kb in size. Then in your User Control Panel, go to your Profile and "Edit avatar." You can upload your image right there.
Signatures can also have images, but that is a bit trickier because there is no way to automatically upload them here. I recommend something around the size of Alyra's, which is 80 pixels high, 350 pixels wide, and 9 kb in size, but I will say you can go up to 100 x 600 pixels for a banner sig. Just remember that smaller images take less time to load.
You can upload your image to a hosting site such as Photobucket, or contact me (e-mail, private message, or however you normally get in touch with me) and let me know you have a signature you would like hosted. I will put it in a folder on the website and give you the URL so you can put it in your signature using the Img tag.
If you would like help with creating either an avatar or an image pic, please don't hesitate to ask; you can post your questions right here, in fact.