1. Upload your picture from the computer to a free picture hosting site like imageshack.us
2. If the site gives an option to resize your image use an size which is less than 800 pixels wide. For eg. Imageshack gives an option for "message boards" of 640x480.
3. The site will give upload your photo and then give a code to use in forums, like the below.
Code: Select all
[IMG]http://img20.imageshack.us/img20/2721/img00071201101171502.jpg[/IMG]
I'll update this post with screen shots when I'm feeling less lazy

People with experience with other websites pls post your "how to"s.
Here is a youtube video of how to.
One more.