Start page problems are usually permissions problems but this doesn't sound like it.

What happens when you try a <img> tag?

Is it an XML-ly valid tag like:

<img src="whatever" />

not

<img src="whatever">

If not, try that, cos the documents are declared as xhtml so it might matter. Other than that, tell us more, give us a link if possible.

When adding remote content like images or Flash to your start page or even when linking to separate Javascript or CSS files you need to make sure the path to it is as though it were being loaded by the start_main.php in the /forum folder and not the start_main.php in forum/forums/WEBTAG/

Other than that what they said ^