Assuming the dimensions of the face match the dimensions of the image (to prevent stretching) and also assuming you're using cycles render, simply
- With the cube selected, press tab to enter edit mode
- Press "u" and select smart UV project. Make sure you have the faces selected.
(keep settings the same, just click "ok")
- Head on over to the UV image editor
- Make sure you're still in edit mode, and you will see the faces unwrapped
on the image editor like this:
- Now add your image by clicking "open" at the bottom left and find your image
Rescale the face to fit the image by selecting it (yours will have an image):
In the materials tab, click "use nodes" and on the color, click the dot next to the color and select "image texture"
- Open image
and you're done.
Here's an example :
Here is the material tab:
When you click the dot, select image texture:
If the UV unwrapping worked out, it should appear in rendered view
(note: it will not appear in solid viewport; switch to rendered or texture)