The image you are loading and the blender scene are not the same size (in pixels)

Blender will not auto-size your images to fit within the compostion.

To know how big your image is press <kbd>N</kbd> in the UV/Image editor to bring out the side panel.

[![enter image description here][1]][1]

If you load an image that is larger than the composition, you will only see a portion of the image.

[![enter image description here][2]][2]

If you add a scale node and set it to fit, then your image will be scaled to fit within your composition.

[![enter image description here][3]][3]

Note that if the images are a **different aspect ratio** than the output, you have three choices: 

 **stretch**: deform the image to match within the aspect ratio of the container)

 **Fit**: keep the images proportions but fill the remaining either with a transparent background or with black (if the "Use Alpha" option is unchecked)

 or **Crop** Not stretching the image, resizing to fit vertically center cropping horizontally.


[![enter image description here][4]][4]



  [4]: https://i.sstatic.net/Qu4DC.gif



  [1]: https://i.sstatic.net/Wcjip.jpg
  [2]: https://i.sstatic.net/DJPwZ.png
  [3]: https://i.sstatic.net/cQFyl.png