Im currently re-installing my system, and I took XNView as my default image viewer/browser.
(throwing away ACDsee

There are some... ummm... "annoyances".
First, the only thing you could do with the folder pane is selecting a folder.
It would be great if drag/drop move/copy, deletion, a context menu and such stuff were available as well.
That brings me to the 2nd thing.
Some more Windows Shell Integration.
Most other applications of this kind allow showing the Windows Shell Context Menu
(eg. wenn right clicking on a file/folder while holding Shift and/or Ctrl Key)
by utializing the IShellFolder::GetUIObjectOf and IContextMenuX COM interfaces.
This feature (depending on your sources/build-structure) shouldn't be that hard to integrate (done such stuff myself),
thus I'd like to kindly request it for an upcoming version

Another thing is Drag/Drop DropSource support for the folder (pane)
(corresponding to what I mentioned above).
You appearently implemented IDropSource stuff for the file-list (files) already... it would be great if you made it available for the folders (pane) too.
[Edit]
Almost forgot.
Giving the Mouse XButtons (WM_XBUTTONxxx) some functionality would be also great indeed.
(Like Prev (Back), Next (Forward) functionalities)
Just suggestions

Keep up the great work (hehe, egoistic, isn't it?)
Nils