Hi,
You could use CSS to specify background images for the links and then set the font-size to 0 and the color transparent on the link. You'll also need padding on the link to display the background-image (icon). For instance:
#toolbar a.logoutlink{background: url('exit.png') no-repeat 5px 50% transparent; font-size: 0; padding: 12px; color: transparent;}
You will probably need to adjust this a little bit to get it exactly how you want it but it will get you started.
Also, please note that if you set UseIconsForAdminLinks to true on the StyleSheetCombiner control, that all of the normal admin links (Administration, Page Settings, Add Page, Edit This Page) will only show icons.
HTH,
Joe D.