It is not possible to use Add-On "Import Images As Planes" after a scene becomes slightly more complex than startup scene.
After selecting an image file and clicking "Import", plane object with image does not appear in Blender scene.
Import menu is also different. Here is comparsion:
Here is file for testing:
For me, this bug appears with any complex(not so) .blend file. This bug appeared many versions ago.
Blender 2.77a, Windows 10, Windows 7