Viewing jpg in digicam when resized with Photoshop doesn't work

I tried this with 3 digital cameras, a pentax, a sony and a sanyo. I
always had the same problem. I took a photo taken by the camera,
resized it in photoshop, saved it to the memory card under the same
name. After this, it was not able anymore to view the picture in the
camera, it shows an explamation mark or an error message. I thought of
using my camera as an album to carry thousands of pictures in
display-quality.

I also tried playing with exif. I took a picture with 4 Megapixels,
resized it to 320*240 using my camera. I resized the big version down
to 320*240 as well, opened the 320-picture produced by the camera and
used photoshop to extract header data and attach it to the other one.
It didn't work either, the camera can display its own resized picture
but not the one from photoshop.

What can I do? What's the problem?