Great images can do amazing things to your PowerPoint presentation. But, cheesy stock images are as bad as poor clip art.
The Empower Your Point blog dug up this amusing video on the subject:
There are no golden rules here. In practice I find that any image that is not "natural" usually does not pass the bar: renderings, staged compositions with models (exception: children), combination of 2 or more images (you can do that yourself).
Related posts about this issue here, and here.