dummyimage is in fact made by one of the same guys, which makes the whole exercise confusing, because they appear to have the same interface but with different themes for their marketing.
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