Ad hoc solution: I created a separate content box and imported the picture by itself (without any data/tables on top)... then took a screen shot. The picture is available in the recently used cache of the file, even if you didn't upload it.

Hope this helps!

Sarah - I have recast this as an Idea...there are currently many different menus used to display branding and background and other images which are currently not part of a defined image set for the file. The idea would be to have a single (Tile?) view that would facilitate the management of all the embedded images in a given file which are not part of an image set, including not only deleting the unused ones, but also exporting some or all for re-use in other files.