Interestingly, both agencies feature a number of patent drawings in their inventory (example from Getty, and from Corbis).
Thing about US patents, and all the different parts of them -- including the drawings -- is that, with very, very few exceptions (relating to requirements that if any part of a patent is covered by copyright, a notice must appear near the copyrighted content AND a form paragraph must be used at the beginning of the patent specification), they're not subject to copyright.
Meaning that, subject to the aforementioned exceptions, you can pretty much use anything in the patent database without paying anyone for the right to use it (e.g., here's the patent the Getty image is from, and here's the one for the Corbis image).
Why does this concern me? Well first, I'm kinda curious how many licenses have been purchased for public domain images as a whole, much less images from patents, from these stock image agencies. Second (and perhaps more importantly), it means that I can freely use truly awesome patent images on my website, such as this one, from US Patent No. 4656917 to Mr. Edward F. Van Halen: