Don't go to Media on the menu bar, go to Info.
Under Info go to Administration > Media > then select collection "Headstones' and click 'Unlinked only' enter a surname in the box next to the Search button to refine your result if required then click Search. A list then shows all Headstones that are not linked to individuals.
To link an image click the Edit icon button near the image and scroll down to the field called 'ID' next to an Add button.
Enter the Red1st 'ID' of the person (or persons) the Headstone describes, click Add and save.