File corruption can happen at several points: when the camera writes to the card, when the card is read, during transfer, or on the computer. The best way to find the cause is process of elimination.

Try these checks:

• Put the card back in the camera and review the images there. If they already look bad, the issue likely happened in-camera or on the card.
• Copy the same files to a different computer or open them in a different application. If they’re fine elsewhere, your computer/software may be involved.
• Try a different memory card.
• Try a different card reader or cable.
• Do a full format of the card in the camera rather than a quick format.

Also make sure the camera has fully finished writing before powering it off or removing the card. Interrupting the write process can corrupt the last shots in a sequence.

Because re-copying sometimes works, the transfer path (reader, cable, USB port, or computer) is a strong suspect, but only testing one component at a time will confirm it.