CulturePics provides custom images from the world's cultural collections.
At its simplest level CulturePics is rather like placehold.it or placekittens.com - you type in a specially formatted url and it gives you a random image the size you want. But the difference here is that the images are sourced from thousands of the best images from the world's leading public collections (currently through Europeana and Flickr Commons). You also get to explore by collection and filter by tag, so if you happen to want lots of pictures of dogs from the National Media Museum, or soldiers from the Australian War Memorial collection they're all just a few clicks away. If you're after a particular size, maybe a wide banner or a square picture to fill a particular spot, then you just specify the dimensions and it will find the nearest match and create the bespoke size to your specification.
Click on 'Get this!' and you'll see a range of options for how to use the image, from a simple download right through to embed code with full title and link to the original. This is all presented with clear information about licensing.