Please check your camera setting and make sure that the size & quality setting is on LARGE/FINE and not on RAW + LARGE/FINE. (see attached photo) RAW files are the uncompressed hi-rez base files used in the photo process and require additional software to be displayed on the computer. The files that open normally are most likely ones with the ".jpg" extension, while the non-functional one is a ".cr2". In this case, there is a corresponding .cr2 file for each .jpg, and is the same image in the other format. Please let me know if this is the case.
Those photos cannot be changed with the camera, so you will need to install and use the Canon software DPP (Digital Photo Professional) to convert the existing transferred CR2 files to Jpegs, that your normal photo software will recognize. The program should be on the disc that came with your camera or you can download it from Canon at the link below.
With the camera set on L (large), photos taken from now on will be the normal jpeg files that are recognized by the computer.
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