Heres a 9 patch GUI widget that I made for a friend, it uses a corner texture, edge texture, and a top edge texture (since rotation isnt supported well for images)
You should take a look at “Draw As” Box option on all Brushes. They’re designed to allow nine-slice on any brush on any widget.
https://docs.unrealengine.com/latest/INT/Engine/UMG/UserGuide/Styling/index.html#states
Oh wow, I did not know that existed!