<img src="whatever.jpg" height="something" width="something" />
ie, it's not a net: tag at all - it's simple html.
it _might_ be better to use
<!-- Net:f:something.htm -->
then they can edit the something.htm file (including adding an image as above) but they can also add text, links, and things like that. So this would be a lot more flexible for them. This is the approach I've used in cases like this.)
Cheers
Bruce