Here's how dummyimage.com evolved http://www.russellheimlich.com/blog/tags/projects/dummyimage...
and here is how placehold.it came to be. (The tweets are in chronological order.)
https://twitter.com/kingkool68/status/10582057668
https://twitter.com/iboughtamac/status/10582115943
https://twitter.com/kingkool68/status/10582270354
https://twitter.com/kingkool68/status/10582407098
https://twitter.com/iboughtamac/status/10582447868
https://twitter.com/kingkool68/status/10582658022
https://twitter.com/iboughtamac/status/10583391789