You need to go to the User CP and find you Avatar settings. They have a certain dimension you go by. 70x70 pixels I think. Anyway. That's how ya do it.
As for a sig pic, Stangnet doesn't allow those which is good. They just tend to clutter.
Shrink the pic down to about 25% of it's original size cause the hosting site won't take it's original size as it is too big for their file size for free and it will be way too big for the post anyway.
When you post you paste the file from the host site in your post with
at the back of the filename with no spaces. Something like this
You can use one of the other forums to practice with like a chat or test forum.