I do not know how to put a custom pic as the avatar. I can only pick from the predetermined list but I do know how to post pictures.
You need to first have a website that hosts pictures, host your picture.
I use tiny pic hosting at
http://http://tinypic.com/
you click on browse so you can locate the picture you want on your computer for the site to host. then click on the host image. it takes a few seconds to about a minute then it says successfully hosted and gives a list of info for your image. the top box of information says LINK:
http://tinypic.com/jagr35.jpg (it will have different letters after the inypic.com part) this is your picture's URL. copy all the info that is after the http//:
When you are writting a message here there is a yellow box above the writting space that has triangles to look like mountains when you put your cursor over the box it says insert image. When you click on this on this it pulls up a window box that says please enter the URL of your image, this is where the info you copied from tiny pic goes.
hope this helps I'll check back in a couple days to see if you have any other questions
P.S. I keep a word document just for all my pictures that are hosted
I write a short discription of the pic and then the URL
for example
http://tinypic.com/18ywqd
chestnut and water bow
this way if I want to insert an image in a later message I do not have to go through the hastle of hosting the image a second time.