Given your image-dired tags have similar beginnings e.g

0.jpg;foo
1.jpg;foo:

Now all you enter – in order to mark – images tagged with foo but not foo:, is this:
M-x image-dired-mark-tagged-files foo$

Note the dollar sign suffix

Why is such profane regex noteworthy?
See how image-dired-dired.el preliminary establishes the tag collection:

"\\(^[^;\n]+\\);\\(.*\\)"