When you're using the Project From View unwrapping method in B | Blender Tricks

When you're using the Project From View unwrapping method in Blender, it will automatically stretch the UV's to match the texture size. When the image you're using doesn't match the camera aspect ratio, this results in stretched UV's. In this case you have no choice but to resize the UV's on the X and Y axis manually. This video shows a way to avoid this issue.