Ok, that seems to have eliminated the error message but necessarily resolved the issue.
When I attempt to crop a photo, the page to do so opens with the image X (the standard way of showing an image can not be displayed).
An attempt to view an image results in the same red x box as above.
Resizing an image appears to do so correctly, as I did so and viewed it outside of Power CMS.
Is it possible there is a simple solution to all of these issues?
Are these commonly used functions?