Can't edit source?

For some reason, whenever I try to edit the source of an image, I can an error and my changes don’t get saved. Does anybody know why?

It may be the source cannot be contacted by the server. Even if the link is valid, hotlink prevention and other server-level measures may prevent its validation, in which case, you should link to the page containing said image. Please post the URL and also provide the image ID number in question in case we need to debug it.

When I came across this problem, I was linking to the container and not directly the image. I try to link to the container page instead of the image,