Topic: Better image previews?
First, thanks for this wonderful program. A couple of things could make it even better, though (well, at least for me )
Instead of the nearest neighbor resizing algorithm that you seem to be using now, a biquadratic/bicubic/Lanczos resize would yield a much better preview, especially at smaller sizes. Those algorithms are well documented and already implemented in many open source projects like comic book readers, of which there's at least one coded in every programing language (personally I use CDisplayEx), so code examples abound.
Probably it would be a little too computing-intensive to do the resizing in realtime as it is now, so maybe you could keep the fast existing code for the while-resizing-the-preview part, wait X milliseconds for the pane to settle down to the desired size and only then apply the better quality algorithm. The same approach could be used when scrolling through the images of a directory if you want to keep resource usage low, even though any PC made in the last 4-5 years should be more than fast enough to handle it.
Support for PNG images preview would be very welcome, too. Probably the less costly solution would be to link to a existing DLL (libpng for example, but don't know about license either; maybe to use it you'll have to make the source code available on your site...). Since the Image & Fax viewer renders PNG images fine, probably from Windows XP onwards there's already some system library that decodes them.