This one looks nice in that you wouldn't be embarrassed if it goes live since the images are generally nice looking but part of the appear of YAPHS (Yet Another Placeholder Service) is that it is really obviously a placeholder so you catch it before it goes live.
[1] https://placekitten.com/ (warning: contains fluffy kittens)
[2] http://placezombie.com/ (warning: contains images of simulated violence)