Paint.NET :: Inserting Photos Into Digital Frames?

I have several discs/flash drives with digital pictures frames that I would like to insert pictures into some of them, but everytime I do as the instructions tell me, by first clickling new, inserting the frame, then I go to start and drag the photo over and invariably, the photo turns into a background, not a layer that will easily insert into the frame, a lot of the frames are png, do I need to change their format or what.  It looked so simple on the U tube video I watched and it sure isn't that way when I try it.  I bought a lot of these photo frame discs on ebay a while back and at that time they gave me a simple program to use to insert photos into these frames, but since then I've change computers a few times and lost the program, will do the insertion of photos into these frames.