I'm with Dave on this one: Image is just a more general term.
A photo is an image.
An image is not necessarily a photo. It could be a painting, or any other 2d representation (without getting into the broader definitions of image as a concept, which is a distinct meaning).
If digitial are moving away from the word "photograph", then thats a good thing for everyone, as it implies that its being recognised as a distinct form. However the use of the term "digital image" is something as a stopgap - in 1900 we might have tacked about "photographic images", until we arrived at photograph. Digital will eventually arive at its own word but for now they're forced to use more general terms, and then qualify them.
(Over to Ed our resident expert on the derivation of words...)