Function: image-dired-mark-tagged-files

Use regexp to mark files with matching tag.
A `tag' is a keyword, a piece of meta data, associated with an
image file and stored in image-dired's database file. This command
lets you input a regexp and this will be matched against all tags
on all image files in the database file. The files that have a
matching tag will be marked in the dired buffer. (fn)