Resolve "Add image support for presentation slides."
Compare changes
Now it is possible to add an image in a jpeg and png format to a custom slide.
The presentation with the image:
Images are cutoff if the size does not fit:
All items on this checklist should be checked.
The adding and showing of the image works fine (as far as I am aware), but there is a bug when you go back to the editing page after adding an image. Since this MR was getting big, I allocated that to the new issue #771.
Closes #739