Wikipedia is a source of articles for school and university projects but it can be also be a great source for pictures and wallpapers, just check Wikipedia’s featured pictures page, here’s the page description from Wikipedia
This page highlights images that the Wikipedia community finds beautiful, shocking, impressive, and/or informative. It is the visual equivalent to featured articles and as such even more subjective. Images are categorized by the topic they represent, which is not necessarily what you might expect just by looking at the picture. There is also a category for images with aspect ratios suitable for wallpapering.
There are currently 863 featured pictures.