Is it possible to print a white edible image that will go on a colored background? I don't have my own printer, I'm going to go to Walmart or Schnucks and have them print for me. One will be going on a black background and one will be on red so transparency is also an issue.
No, you really can't put an edible image on a colored background. I suggest rolling out a sheet of white fondant as thin as possible, moisten it with a damp paper towel, stick your image on that. Cut off excess fondant, then place fondant-backed image on your cake.
If I am understanding your question correctly, yes it can be done. I have printed my own image onto icing paper then trimmed the icing sheet close to the image before removing it from the paper. You will have to put it in the freezer for a few seconds so the trimmed image is stiff enough to remove without tearing.
To be safe, you can just put a thin layer of white buttercream or fondant over the area where the image will be.
If you want the icing sheet background to be black or read. Then the letters would actually be transparent which when printed in the image machine no color would print where the letters are. So, as long as your frosting is white, it will work. It is something that you would have to have printed by someone who has an icing imaging machine that is hooked to a computer rather than the type that walmart has.