I've been a big fan of Placehold.it since I first ran across it. It's a service that makes it super easy to dynamically generate placeholder images for wire framing or during development. I decided to wrap the service into a JQuery plugin that will make it easy to generate FPO images at runtime.
You can have an element as simple as <img /> and call $('img').placeholdit(); to generate an image for it. You can also set the dimension of the image being returned in the attributes of the image like <img width="200" height="300" />. And if width or height is set in CSS, it will respect that as well.
Also, by default if the image has a src attribute already set, then it's skipped. You can override this by changing overwriteSRC to true and it won't matter if the src attribute is set or not.
Below are a few examples of it working, so view the source to see the actual code for it.
You can always grab the source from GitHub.