Sometimes when you're prototyping a new website or a new page, you would like to add a image placeholder, just to see how things fall into place.
You can use a service like placehold.it or a small javascript like holder.js, which spares the extra requests to an external server, not that in development this matters much.
For example, in order to generate the two placeholders below, you would only need to write the following HTML code:
<code><img src="holder.js/235x120/#bdeeb7:#97be92"/>
<img src="holder.js/192x120/#b5e9b3:#90ba8f"/>
</code>
Holder has a lot of options, as you can see you can also define the color of the text and of the background, and much, much more.