The images may be in an incompatible (with your Gallery app) format, or they may have been moved from where they were originally downloaded. They may not even exist on the device (there are a few different reasons why).
Try finding the images with the file manager, and selecting them from there.
If they exist, and still wont open, then inspect the format if the image. The name of the image should end with .jpg or .png or similar. I have downloaded images that only needed me to add one of these to the name and then they would work. Other times, I would need to change the format.
There are apps that can do this, but I usually just use an on-line service that is free.
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