I think that overall, an 'insert image' page will be a useful thing to have. Eventually, IIRC, DW is looking into a scrapbook-type feature where users can store albums of images with their accounts. (Obviously, a paid or premium paid feature.) Having other image-inclusion features on that page - even _starting_ with those other features - makes tremendous sense to me.
Flickr's API allows for search by keyword (we already have tags, combining the two in some form should be doable) and limit by creative commons licenses.

I think this would be _awesome_ functionality for many users - the ability to find legal images and have the right code delivered to their DW draft post - and from where I'm sitting it's definitely an attracive feature, even without the scrapbook link - I can imagine browsing my own flickr stream in that way, for instance.