I was googling around and found this free utility (http://greggman.com/pages/thumbplug_tga.htm). It didn't trip my av software and seems legit enough, thought I'd pass it on.
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Ok, been playing with it. Unlike the built-in image preview formats, Windows will not "remember" your preview setting. I've only tested this with a directory containing only TGA's, so perhaps if there were mixed image formats it would remember.
Also - and this is intersting - it doesn't show alphas. In fact, I had a number of images I had added transparency to and then erased some of the visible parts. The thumbnailed image still shows the erased content, so it looks like all transparancy does is "paint over" what's in there instead of actually destroying content. Image displays properly in SL, however.