Maps from OpenStreetMap are licensed under CC-BY-SA, and with appropriate attribution, they can be used freely. Note that the content of Stack Exchange sites use the same license, so they are completely compatible.
The situation with Google Maps is slightly more complicated. There are two applicable documents -- the Terms of Service and the Legal Notices. Take from these what you will.
I will study these in more details later on, but from a cursory analysis it seems to be okay to include an occasional screenshot of the map. The usual IANAL disclaimer applies.
For images, the best place seems to be CC Search, which incorporates a few other databases with appropriately licensed content, including Google, Flickr and Wikimedia content. One should comply with the license conditions, mostly related to attribution, when using these images.
All being said, in this case the maxim "It's easier to beg for forgiveness than ask for permission" is a good policy. It's easy to argue that usage of images, even copyrighted, are not used for commercial purposes -- Stack Exchange sites do not generate revenue of any sort from the site itself, nor do they charge for their usage. Besides, the DMCA takedown procedure provides Stack Exchange Inc. with a "safe harbour" against copyright infringement claims, and if a copyright holder complains that their content is used inappropriately, Stack Exchange can remove the content in accordance with the claim (if it is deemed valid, of course), and not be a subject to further liability.