How do I add a picture at the bottom of my post?
Assuming you mean like as your signature, you go into profile -> modify profile -> forum profile -> signature, then within your signature you post forum code with a link to the picture you want as your signature :D just make sure the picture you want as your signature complies with the forum rules
There is also a post count minimum before you can post pictures. This may also apply to the signature.
You mean like my weebl?
You could host it somewhere like http://imgur.com/ (http://imgur.com/)