By reselling images to clients, most agencies and design firms are probably falling into a grey area in terms of violation of these license agreements. This is a very interesting legal issue that must be causing conflicts on the order of thousands of times a day. We all buy stock and use it in work. That's what stock images are intended for. As an agency, you are open to a potentially huge liability from either of two points:
The stock agency could aggregate the infractions of all the stock images that were in violation of the terms and demand recompensation. To my knowledge this has not happened - but we are not a large agency...it would not be in the best interests of longterm client relations...
Any client could sue for damages for images that were ostensibly sold to them fraudulently. This is also unlikely, because the client is unlikely to know the terms of the license - unless the stock house approached them, and as broad "royalty Free" licenses are most prevalent with images, it would prove a challenge to track.
To avert the liability, you should probably buy the stock images in the customer's name, OR put clear verbiage in your agreements with clients that the rights to these broadly licensed stock images may be subject to certain legal encumberances.
We negotiated a contract with a big major brand client recently, the boiler plate contract initially demanded all sorts of broad copyright and licenses be granted as defacto. We lined through all such broad statements and demanded that each project description define the specific copyrights to be transferred, and that all stock images, soundtracks, movie clips, personages, likenesses, copy, intellectual property, processes, patents, and trademarks be specifically defined AFTER the project completion. It was not simple.
I concur with Andrew, definitely seek legal advice for the broad contracts.