Well, learn something new every day…
“Mozilla browsers refuse to follow file: links on a page that it has fetched with the HTTP protocol, so that the page's own URL is an http: URL. When you click on such a link, nothing happens. The purpose is presumably security: to prevent a remote page from executing a program on the visitor's computer. The file: links work on Mozilla on pages that are local files on the user's disk.”
Houdini's embedded Mozilla browser gets the help from a mini web server running inside Houdini (it might sound convoluted, but it makes some important things easier/possible).
(For what it's worth, either or <img src=“file

//c

foo.jpg” /> would have worked if not for this.)
To refer to images on disk, try this:
Create a $HOME/houdiniX.X/help directory, and inside create a myimages (or whatever name) folder for your images. Then reference them using .
Houdini now overlays the contents of any HOUDINIPATH/help directories over the standard help. This is a fairly recent change, so if you're using an older build it might not work.
Hope this helps!