(OK, right off the bat I'm bending the "one issue per post" guideline a little, but these two things really do go hand in hand...)
"Avatars" are an option available only to users with login ids.
"Signatures" however can be done with or without a login id.
The Avatar is a small image that appears in your posted messages next to your name.
If you wish to create or modify your avatar, go to
http://www.network54.com , log in and in "My Account" area click "Edit Account", then "Personal Photo". Avatar images uploaded are forced to an image size of 70 by 70 pixels, so unless your image is already exactly that size, it's going to get distorted. To change image sizes you generally need some software like Photoshop or Photo Editor etc.
The Signature is some personal image, link, and/or message that appears at the bottom of posts you make.
If you HAVE A LOGIN, and are logged in, on the page where you create your post, at the bottom, there is a small expandable area where you can paste in any text or html code that can optionally be attached to the bottom of any post you make while logged in. This signature area is what most folks use. In it you can write whatever you want or place whatever html you want (such as the url to link to a small image of your car or personal homepage etc). Then each time you post, it's already in there, and you can just check/uncheck the box that asks whether you want it attached to that particular message.
If you DO NOT WISH TO HAVE A LOGIN, the way to have this would be simply to take the very same information you would otherwise have put in that signature box, and instead save it to a plain .txt file on your computer. Then whenever you wish to post a message and have a signature image/link/phrase appear, simply highlight the .txt file contents, copy to clipboard, and paste in at the bottom of the message you have written, before clicking "Post".
To end users it will look the same either way. It's just down to whether you would like to have a login or not, but again, having one does offer additional benefits, and you then don't have to paste the signature in every time you want it.
I often do it both ways. If I'm logged in of course the sig is there as a pre-arranged optional include, if I am not logged in, I also have it on a txt file on my machine so if I'm feeling too lazy to log in just for that at a given moment, I'll just paste it in and keep on rolling.
There ya go!

Peace, -Mac
Thought for the day: Today's mighty oak is just yesterday's nut that held its ground