Hammer has a @placeholder tag that you can use to easily generate placeholder images.
<!-- @placeholder 400x300 -->
Hammer will generate an image tag in place that references an image from placehold.it and uses that URL as the src attribute. The images this produces look like the example below.
By default the image will display the image dimensions as part of the graphic. You can replace this with your own embedded message by passing your text string to the tag, as shown below.
<!-- @placeholder Hammer+Time -->