I uploaded a bunch of pictures into an album viewable only by administrators. Then I linked a bunch of them to an album that can be viewed by Everybody, using the album keyword feature.
When a visitor clicks into that public album he gets "No image to display." When an administrator clicks into that public album he sees all of the linked images.
What do I have to do to enable a visitor to see the linked files in the public album?
Linked files respect permissions, so what you're experiencing is expected (and intended) behaviour. If you need the linked files to behave differently, you'll have to modify the core code.