Actually nearly all (maybe all) of the Alice library models do use .png images for textures.

The only place I've seen this is required, however, is if you are going to use alpha level transparency in the texture (rooms in windows, for example). I've never been able to get that to work for any other format - though I haven't tried very hard.
For the two or three programs I work with a lot, .png has the most consistant implementation so if I can't use the format something comes in, that's where I tend to go. However I have used .bmp quite a bit and .jpg some. Don't really recommend .jpg for anything other than quick work, however. Sometimes I get funny color effects from the compression when I scale objects.
Speaking of scale - make sure you at least roughly scale your texture to your object. I find 64 pixels per "Alice meter" is a good place to start, but for more complex textures I recommend you experiment a bit to see what works best as you scale the object - and please do share your results.