I'd like to point out that Episerver does not support the Edge browser, according to system requirements.
That is a very valid point Bob :)
Just for the fun of it I tried reproducing in IE11 but from what I can see TinyMCE does not support moving images using drag and drop in IE11.
Edge (41.16299.726.0) freezes after a while when moving images inside the TinyMCE editor, hogging one CPU core and as much RAM as it can get a hold of. It seems to be related to the version of Tiny (4.7.13) currently used by the latest Episerver package. I can reproduce it on TinyMCE version 4.7.13 but not in 4.8.5.
Anyone that can confirm and give information on plans to update the TinyMCE version in the Episerver package?