Absolutely, but do keep in mind that I'm talking about the current Opera version (30). Recent others use the same approach but I don't know how earlier versions (such as 12 and such) did it.
Every theme is basically a zip archive which contains the image and one or two ini files which provide further information (name of the theme and author, name of the background image, position, etc.). So, if you go to: \Users[your name]\AppData\Roaming\Opera Software\themes then you'll see all the themes you downloaded as a zip archive. Just open the archive and then open the image to view it. I'd recommend using IrfanView as image viewer because it's quite versatile.
Hope this helps!