I'm Trying to make a poster in a room and when I insert the Image Texture It splits diagonally. I am making a Building FULL of Pictures And Posters.
Need to see a screen shot of your uv mapping, but if you select the plane and use 'U' - 'reset' and then 'Unwrap', do you get the same issue? If you unwrap a single plane usually it covers the entire image in the UV/Image Editor. Do you have a seam there along that dividing edge?
edit: If you can, please look in your addons under user preferences and find "Import Images as Planes" and turn that on - and then choose 'add mesh' and then choose 'images as planes' - UV mapping is already done, as well as aspect ratio matches the plane width/height automatically. I use that in all my work as a basis to start painting with.