I tried using my Canon camera as a picture viewer, putting some downloaded jpegs on it- the pictures didn't show up at all when I tried to review them, only the pictures I had taken with the camera. Renaming the pictures to have camera names like DSC_0025.jpg made the camera show a question mark icon for the picture at least.
A little searching later I discovered a tool called paint.net, which saves jpegs in the proper format so the Canon will like them. The other trick is to re-size the canvas the picture is on so all dimensions are multiples of 8.
paint.net itself is somewhat interesting, quicker to load than Gimp but not so much easier or more intuitive or like Deluxe Paint that I'll use it for anything else.