Don't worry about the placement of the text too much; that's kind of a done deal. I guess you could go for a caption underneath the frame like at a museum if you really had to go off-centre, but my decision to place the text in the middle was scooped straight from OS X. I think of it as more of a convention of the medium than a choice I really had control over. Of course, you could try going with no text at all, and try something more thematic; most image viewers (XnView, ACDSee, IrfanView...) expect the user to at least partly rely primarily on colours to distinguish what's what (GIF usually ends up green for some reason) and as for content, a lot of them also go with what the file's best at—JPEG is for photos, GIF is for simple graphic art, and so forth. It'll take a bit of cleverness to come up with a scheme but I'm sure it's possible.
However, you do also have something of a theme going with your current trend of putting document types in the top-left corner. The most consistent thing to do would be to put a scrap of paper hovering over the frame.