You can put UseImage="true" in the <portal:mojoPortalLink control. The problem is that there isn't a way to specify the image so it uses an image from the \Data\SiteImages directory. You can replace that image with an image of your choosing (it must have the same name) but it would be overwritten on upgrades.
You can just create a link to the mojoPortal website.
<a href="http://www.mojoportal.com"><img src="mojoportal-logo.png" alt="Powered by mojoPortal Content Management System" runat="server" /></a>
If you did this, the image would have to be in your skin directory but then you could use any image you want and it wouldn't be affected by upgrades.
You can download a mojoPortal logo from my website if you would like. There isn't one on this site. :-(