I don't think there's any content provider in his right mind that would NOT allow you to put you URL on the images. With all the content theft going on in today's market, having your URL on the images tells the content provider where the content is coming from. A big plus in keeping up with who has a legit license for those pics and who may not.
Of course, you can't put a copyright on them, but I'd like to hear of any content provider that doesn't allow a URL.
