I did some playing around with my Canon 20D tonight, and I found a way to write files properly back to a flash card so they'll show up on the camera. Most cameras are picky about what they'll display (at least Canons are), but the magic sauce appears not to be *too* tricky to make.
First, some ingredients:
1. JPG files taken with your camera
2. Picasa 2.x
3. Knowing the pixel size of the "small" mode for your camera
I found #3 by shooting a 'small' shot on the 20D. It's 1728x1152 (and the important number is '1728' here).
The only thing to do now is to "Export" from Picasa, with a custom size (1728 pixels, of course), and with awful quality (try 20%). Copy the resulting files back to one of the DCIM folders on your camera.
My results? 460 files in 25MB. All nicely viewable on the camera's LCD screen. And all your old 16MB cards should hold about 300 shots. Been a long time, huh?
See this dpreview thread (where you can watch me think the problem is harder than it is, and then figure it out) for some more details.