I'm not sure what the allowance is for registered members, with regard to attachments; but the procedure is pretty much the same on vBulletin sites. When making your post, look at the icons site the text box. Click on the one that looks like a picture of the tree, then select the file.
The number of attachments you'll be allowed is quite limited; so find a picture hosting site that you like (ie photobucket), and use the IMG code provided.