Placeimg.com style mock assets
The ability to designate a box as a dummy asset of a certain theme so that it displays an appropriate image (roughly the same size and theme). For example, when creating mockups on the web I've been using placeimg.com to show temp assets. If I had a nature web site and wanted to show a 340x340 image I would just reference https://placeimg.com/340/340/nature and it would show a random nature image for the demo.
William Tsui commented
keywordimage.com is another good alternative for specific keywords
Thomas Kelly commented
https://keywordimg.com/ is a great one too