drag-and-drop "replace" menu
There are only 2 reasons why I would do this.
1)  I'm trying to add a picture and forgot it already exists.
2)  I want a picture to be a PNG sprite / animated GIF instead.

So the "replace" options are
1)  Cancel
2)  Replace existing picture with PNG  /  Replace video with a GIF


What happens when importing multiple images?
Ideally each menu takes turns popping-up, and all non-duplicates proceed in the background.
This could pause the "addThisImage" function, so the overall process does not finish until all menus are answered.
