When you are using an SLR to look at images you have taken on the back of your camera, set your camera to not autorotate the images. That way you can see the image fill the entire LCD instead of part of the LCD with big black bars on both sides.
On some cameras you even have two options: on the camera, or in the image itself.
In this case I set autorotate ON in the file, but OFF when reviewing on camera (the middle option).
You will find thise fuction either in the playback menu of your camera, or in the settings menu.