Oh, I see. Mystery solved then.

Certainly I created additional levels which I removed when I deleted all the content, leaving only the first level with the background image.

I hope it can be fixed easily. It's not a big problem, but it's more annoying when the imported background image has a large size, like the one I used.

Puybaret, if for some reason the fixing implies many or complex changes, I guess at least you could make a simple substitution of the actual image file for a dummy image file (0 bytes or 1x1 pixels), that way the wasted space is released.

Thanks and regards.