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 N in the UV/Image editor to bring out the side panel.

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

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

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.

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 N in the UV/Image editor to bring out the side panel.

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

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

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.