Copyright issues. Yes, a screenshot is categorized as an infringement because you are copying without permission. However, there is some legal consensus (at least in the U.S.) that thumbnail-styled reductions are permissible as fair use when used for informational purposes--for example, search results, historical timelines, etc. For example, your Top Sites screenshot doesn't need permission because it is a series of thumbnails of other sites and that's apparently considered an acceptable practice within the industry -- obviously Google (Chrome), Apple (Safari) and Microsoft (IE) (all of which offer 'top sites' features) don't seek permission and there is no reason for you, as well.
As for the other two screenshots ... The Dear Rich Staff regularly uses screenshots without permission in our books because we believe that it is commonly accepted as a fair use, is justified by the recent thumbnail decisions, and perhaps most importantly, no website, unless they are being portrayed horribly, would want to hassle someone who is ultimately promoting their site. It would be a bad legal, tactical and public relations decision. (That said, the more screenshots used from one source, the more likely you might run into a problem. For example, one exception to this -- and it wouldn't apply in your case -- might be if a company wanted to provide the exclusive manual on how to play its video game or operate its software program. In that case there might be a stronger basis for pursuing those who copy screenshots).