this question just came up as we recently bought content from image stock portals. Many of those altered their license agreement in favor of charging more for using in mobile apps. So instead of using their standard licenses, you need to pay an "extended" licenses which multiplies the fee easily by 5-10.
That doesn't make sense as the mobile device is just a smaller browser and protects the content even better than a desktop computer.
Are those stock agencies allowed to do that, and is it legal at all ? I am not a lawyer but I would even risk to go on with the standard license and wait to be sued in that matter.