Renderhjs, you can do the things you describe your thumbnail viewer doing, in windows, better and more efficiently. From what you're writing it seems you're using (or are accustomed to) XP SP2. Win7 thumbnail features are VERY GOOD and go beyond anything that can possibly be done in a 3rd party app. I really recommend you check it out and learn the capabilities. I'll jot down a few notes to help things along:
First of all, you really should use project setups (max and maya both have very good support for this). Dumping all files in the same directory is bad practice no matter what. So you have a solution that filters out anything but images, well, what if the file you're after is not an image? Then you're back to square one and really need to sort files better.
That said, however, windows can categorize according to file type. Please note that I don't mean sorting, but categorization independently of sorting (which can still be set to time, name, etc).
If I ever need to do something complex like batch-conversion, I'll just use my 3rd party thumbnail viewer, same as you. Having a good windows thumbnail viewing setup doesn't exclude you from using 3rd party solutions in the rare cases that you may need them. And if you need to use them a lot that sort of thing can be scripted quite easily in the native windows explorer interface (passing multiple selections to batch-conversion for example, easy to setup, and I'm sure your image viewer supports this stuff in command line like most good image viewers do)
You mention 400 image file formats. For my personal needs I only ever use a limited set of formats (if someone here sends me PCX I'll tear him a new one), but regardless - windows has something called Windows Imaging Component now. That's basically like video codecs, but for images. This means that extended thumbnail support is no longer a hack (previous DDS thumbnail viewers would compromise system stability in XP) but an actual integral and easily 3rd-party extentable feature of windows.
So, not only can windows nowadays do all that I require of thumbnails (and needless to say I deal with thumbs a LOT), but it can do things that no third party app can do, namely it's integrated into the core, so all file dialogs support them.
Say I want to overwrite an image in a folder with what I'm currently doing in photoshop:
Using windows explorer thumbnails:
double-click the right thumbnail.
done. No waiting, as thumbs are loaded immediately.
Using a 3rd party thumnail viewer (here we go):
then right click an image file (thereby losing the content of the filename field, which in many cases is very unpractical)
start the thumbnail viewer
click the thumb you want and copy its filename (does xnview only copy the name not the extension?)
close the thumbnail viewer
focus on the filename field in the save-as dialog
hit enter, or the save-as button.
done. (there's also delays for waiting for the thumbnail app to load)
This type of should happen a lot. You should be browsing images to actually use them for something, not for fun, meaning you are opening/saving from an existing app, like saving a texture from photoshop or opening a diffuse map from a 3pshader material in max (click the diffuse slot then double-click the thumbnail; super fast). This happens a lot and your 3rd party app solution does nothing to help it; rather the opposite.
Because you can't (safely, to my knowledge) replace file dialogs in windows, and because windows (recent versions) has superb thumbnail support, I now only use 3rd party thumb viewers in rare cases (and since it's a rare case, optimizing efficiency is pointless anyway).
The above should objectively demonstrate how windows explorer thumbnails are far from the "wrong" choice and are indeed very good. Just make sure you truly explore the new features in win7 instead of assuming it's like XP.
On another note xnview < irfanview, but that's another topic