heh, this is one issue that I'm guilty as charged for. And I'm one of the more rigid advocates here on the topic of copyrighted content.
I use images from Google all the time. I use them in my videos and I use them in my thumbnails. The thumbnails issue is probably a bigger one than the videos issue. That said, my usual method is to use them to spice up otherwise stock gameplay video content by giving my editorials some flair. An image might be on the screen for a few seconds, just to provide some context. Given that my content is primarily news editorial in nature, showing imagery related to the story that I'm telling and keeping that imagery a very very small portion of the overall presentation, I feel that I've got a pretty good fair use defense if it ever came down to a battle. I don't credit sources because I might use 20 images or more in a video, but then with fair use crediting the source doesn't win you any points. Source credit is more of a licensing issue as part of the license agreement.
I know that Google does the usage rights thing, but the internet is such a repost heavy place that it's difficult to know whether the usage rights are valid. I can upload a video of anything to YouTube and mark it as creative commons, even if I don't have the license rights to do so. Hell, I'm looking at licensing the Masterpiece Theater theme song (1600s classical music, so the piece itself is public domain) but I'm mildly concerned that because it is so recognizable, the version I want to license isn't actually under the right license. >.<
I probably should credit the images I use in my thumbnails though as those are much more major and outward facing. I mean, I quite literally use the image to draw people in, and I suppose that's a failing on my part.