This is something that's set up by the forum administrator. Typically, it's turned off and UBB Code is enabled (as is the case for this forum). UBB Code is a simple way of doing things that you might want to do in a forum like post an active URL or modify text formatting. You don't have to know HTML code in order to use it.

If you just want to put a direct link to your web site in a message, you can do that by clicking on the "Link" button (the one that looks like a globe and a paper clip) in the "full" message editor and enter the URL in the pop-up screen, like this:

Elena's Web Page

It appears that the button to insert an image is disabled here, probably a good idea to discourage what might appear to be an advertisement (as if a link to your own web site isn't - )