I've just implemented a config setting to allow external images. After the next release of mojoPortal you can add:
<add key="Forum:AllowExternalImages" value="true"/>
to your user.config file.
However, allowing external images to be posted in the forums can potentially be a security risk. Also if you use SSL on your site (which everyone should these days), external images can cause browser warnings about the page containing insecure content.
This change is in the source code repository now.
Hope that helps,