There was a time in web development when it was common to need a spacer or placeholder image of a given size. It’s a shame, then, that the use of PlaceKitten is not more widespread:

A quick and simple service for getting pictures of kittens for use as placeholders in your designs or code. Just put your image size (width & height) after our URL and you’ll get a placeholder.

Like this: http://placekitten.com/300/250, which produces a kitten 300 pixels wide by 250 pixels high:

Enjoy.