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Windows Bitmap (.bmp, .BMP extensions)
Encapsulated PostScript (.eps, .EPS extensions)
Microsoft Icons (.ico, .ICO extensions)
JPEG images (.jpg, .JPG, .jpeg, .JPEG extensions)
KRL images (no extensions, don't know about this format)
PNG pictures (.png, .PNG extensions)
PNM ("portable anymap") pictures ( pnm, PNM, pbm, PBM, pgm, PGM, ppm, PPM, 
pbmraw, PBMRAW, pgmraw, PGMRAW, ppmraw, PPMRAW extensions)
TIFF images (.tiff, .TIFF, .tif, .TIF extensions)
XWindow bitmaps (.xbm, .XBM extensions)
XWindow pixmaps (.xpm, .XPM extensions)
XView format (no extensions, don't know about this format)

I haven't actually tested this, so it is probably worth a trial.

And, as shown above, pnm is a different format (format group, really) to png.

HTH
Brad