First off, you either upload the picture on a website like photobucket or Image shack, than than should give you a link with thew [Img] tags around it, so just paste it here.
If you DON'T want to use those websites, you can also upload a pic using the forums, look below the writing field when replying, there should be a place to attach up to 2 files, so just browse for your files, than click post.