As far as I know, I couldn't find another way to make the source of an image appear than adding a text area above an image, then move it to the bottom and edit the style so that sourcing of images in a presentation appear legit.
Instead, it would be nice to simple have a field in properties that is called source or reference, add whatever texte there (url, name etc). Then, a simple option to toggle on and off to make the source appear on all images of the presentation with a preselected style would be very valuable, saving lots of time (better spent on the content instead of the formatting).
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While I disagree with the "simple" toggle on/off (consider position, font, color etc.) the idea of having source and license attached to the object has some charm. What do you think, Regina?
Good idea, independently from whether it requires definition in the file format.