I am trying to make Rotated Transparent Images as Main Menu Button backgrounds, but seeing as the Image is a square with transparency, the result is that the images overlap. (SEE PICTURE BELOW)
How would I avoid this?
Play Button is Selected but overlaps Option Button
What strange way did you use to end up like that? Refference:
Simply changed the Angle of the vertical Box. Works as expected. In short do it the Proper way 
Ahh I see, I used one vertex box for the whole screen, so now I get it!
Thank You!
Now my problem is the whole image is not showing and is cut off:
Thats a know issue currently it will be fixed in 4.17 see here:
https://forums.unrealengine.com/showthread.php?148183-UMG-and-Slate-New-Clipping-System-in-4-17
What you can do as a workaround for now is to Translate your Vertical Box and make sure the Dots that usually are your Corneres and sides stay inside your Canvas. If they go outside it will start to Cut out the rendering part. A Performance improvement to not render anything offscreen usually.
Refference Image: