Rainer T , Jan 10, 2007; 07:36 a.m.
with those cameras I've seen the main point is, that an image that is brought into the camera from a computer is not longer the original image ... it has been edited. Thereby the original fileheader including the original EXIF information (including makernotes) are gone, or at least modified.
The camera now feels no longer confident to display that image.
Another point is, that the image doesn't match the directory-structure and naming scheme used by the camera.
One camera (my first P&S) would display modifed images, if I eqquipped them with an original EXIF-header, and generated a new thumbnail. This was my old Kodak DX4530. Other cameras ignore the images under the same circumstances.
So, to be able to show images to friends using a spare SD card, you need to have the originals. If they are placed on an SD card containing the correct directory structure, the camera will (very likely) not ignore them.