When reloading a ZIM, images are broken if their paths in the ZIM have changed

[audiodude]

This might be a weird edge case that isn't a problem in practice.

Steps to reproduce:

1. Open a ZIM file created with mwoffliner
2. Re-create the ZIM file using mwoffliner, the image paths have changed
3. Go to "Configure" and select the "new" ZIM, at the same path and load it

Expected result

The ZIM is reloaded

Actual result

The ZIM appears to reload, at least the main page is shown, but images are broken.