Mh, if I select everything in WYSIWYF edit mode on one page which contains images and paste it into another page’s WYSIWYG editor, all looks fine for a moment, including the pasted images.
However, if I then save the target page, suddenly all images are only shown as broken links.
Is this a known issue? I’m using the XWiki 10.11.8 Debian packages.
From a technical POV I guess the images are relative links and not absolute one (they were not added with the WYSIWYG editor itself probably?). When pasted in the other page, it’s not seen as a refactoring, so when you save you still get the same links pasted and they fail because they’re relative and no longer point to anything.
As far as I remember, the affected screenshot actually was originally uploaded using copy/paste (pasting the image alone from the screenshot taking application) into the WYSIWYG editor.
I understand the potential technical issues behind the symptoms, but the current behaviour is nevertheless wrong / a bug - especially as immediately after pasting everything looks fine including the images, and only after saving the document they are lost and replace by “broken reference” icons. 
So if it’s not known so far, I shoud probably report a bug in JIRA.