I've just found that test cases that contain images and are copied from one project to another are unusable by Testers in the destination project unless they have rights in the original project. Testers get broken images.
Troubleshooting finds that, if the Tester requests the image directly via the URL embedded in the HTML, they get re-directed with the message "The requested project does not exist or you do not have permission to access it".
As there is no history of where the test case was originally copied from, it took me a while to find the right project and give the user rights. Once I had done so though the resources were visible again.
This is not a usable work-around.