My parents' work camera is a Nikon 30x Wide Coolpix. I was importing pictures from different SD cards to the computer last night and I imported some work photos. I later was told to put them back in the camera. I tried for about three hours, placing them in different folders in the sd card. When I inserted the said cards in the camera, I got the message "file contains no image data." However, when the card is in the computer (an Imac in this case), the pictures read perfectly fine. I absolutely want to be able to be able to read the pictures on the camera, for my parents need it for work starting Monday. Can anyone help me ?
Try formatting the card in their camera and taking one shot. Then put the card back to the computer and copy the images to the same folder as the one image taken with the camera. Make sure that You copy only unedited pictures shot by that camera and that there are no other extra files.
Each camera has a specific folder (and naming convention) for its files.
You have to put them exactly there, and with their original names, not anywhere else.
Just shoot any picture now, and check where in the file structure on the card it ends up, so you know the proper place (and naming, in case you changed file names).
When you mix files from different cameras in the same card, the camera maybe unable to read them properly and give those kind of errors or as in your case, not showing images at all.
It happened to me twice at least, when using a card from a compact camera with a DSLR, when I put the card back in the original camera it wasn't able to read any of the images. Back in the DSLR it showed OK its images but not the original camera ones.
Cameras from different manufacturers most probably have different folder names. Files not in the appropriate folder may lead to the camera not showing them. Most camera manufacturers nowadays use to follow (DCF) Design rule for Camera File system and latest DFC specification in PDF
Check the correct naming by formatting a card and taking some pictures with it (as suggested in other answers).