well first look at the extension. is it jpeg or tiff, targa
jpeg: you're fine it doesn't contain an alpha channel.
tiff or targa: to be sure check the image in photoshop or any compositing software.
sometimes the viewer in your operating system (win or mac or whatever) will show you the image cut out. other wise you will have a nice rectangular image...
maybe this could be the answer to your other question... *shrug*