Assuming you've already got it as some sort of computer file (drawn on the pc/scanned into it), next thing is to find a place to host it. For example, I tend to use my own site, however most people lean towards photobucket or possibly tinypic. (I'm loathe to mention Imageshack but it's TECHNICALLY an option.)
Either way, once you got it hosted somewhere, say at
www.imagehostingdudes.org/boredunderwear.jpg (not real) or something, then you'd just wedge that address in between IMG and /IMG tags and you'd be rockin'.