Just shooting out a quick thought before I forget again -- it would be helpful in a number of places to be able to reference "internal" software-provided images from wikitext.
A primary example is for help pages and inline explanatory text in parts of the UI which we want to be able to customize. This is something that's coming up a lot in things the Usability crew is working on -- folding the wiki markup cheat sheet into mini-help pages accessible from the editor, having screen shots for skins and features so users can see what things are about before enabling them, etc.
One "simple" implementation could be to slip in a custom FileRepo, so they could be referred to just like other images:
[[Image:help-markup-example-thumbnail.jpg]]
To be slightly more complex, we could give them a unique sub-namespace of some sort, or strip out hardcoded extensions so that types can be more flexible.
Skins might want/need to be able to override certain names, too... and we might consider about needs for localized images -- whether RTL/LTR only or actually making translations available.
Any thoughts?
-- brion