When creating sketches and early mockups, it's often useful to draw image placeholders instead of including actual photos, icons, ads, etc. in the layout. The new Placeholder auto shape makes this a snap.
The grey box mode is similar to the webby placeholders made by http://placehold.it, while the white boxes with an X across them are a more traditional graphic design look. You can also change the label of the placeholder to any string you like.
You can download the extension from here: http://bit.ly/9fS7Fq