Go to a layout based on the image table, and script a search via a related field for the a value greater than Zero in the related billboard table.
This will bring up a found set of all images with a related billboard.
Next Show Only Omitted Records, then delete that found set, which will be all the image files which don't have related billboards.
Thanks wiill give that ago.
Remember, when searching via a related field, that you need to enter something like ">0".
Searching on "==" via a related field often will not always return correct results. That's why I specified searching for a value greater than zero, to be sure a real value exists in a real related record.