How do I link to an image in the tpl_main_page.php template?

"<? echo DIR_WS_TEMPLATE; ?>" inserts the stem of the template's URL into the page.

So, if your image is uploaded within the template's "images" folder, then use code such as the following...

<img src="<? echo DIR_WS_TEMPLATE; ?>images/logo.png" alt="logo" width="300" height="87"/>