The images served by /random come from a local bank of 320 photos by 47 authors, sourced from Wikimedia Commons. All of them are licensed CC0 1.0: public domain. Attribution is not required, but here it is for every photo, with a link to its source page.
Filter by category with ?cat=, for example /random/800x600?cat=animals.
?cat=animals 39 photos?cat=architecture 51 photos?cat=city 24 photos?cat=food 39 photos?cat=landscape 58 photos?cat=nature 51 photos?cat=tech 11 photos?cat=texture 12 photos?cat=water 35 photos