If any of the following conditions are not met, address them and recheck to see if the problem still persists.

• The DCIM folder structure on your SD card is intact. (As described by @twalberg.)

• You are following the file-naming pattern of your camera. All filenames must be capitalized. You may want to double check with Windows or Linux.

• The files are of types your camera supports (eg, JPG, CR2).

If the problem still persists, then the problem is most likely, that the images have been modified since leaving the camera, as others have pointed out (@HarryHarrison, @salmonlawyer). The most likely culprit is the image-transfer software you are using. Changes it may be making include:

• Rotation of the image. Use a program, such as exiftran to losslessly rotate the image back to landscape orientation.

• Removal of the embedded thumbnail. Cameras typically display the embedded thumbnail, not the image itself. You can try using exiftool to embed a new thumbnail. Or you can save as a new JPG in landscape orientation with a new embedded thumbnail.

• Conversion to an unsupported file format. No other option but to save a new JPG in landscape orientation with an embedded thumbnail.

In the future, use a standard file manager to copy the files directly from SD card to your hard drive. Image transfer software often do not keep original data intact.