Showing 3D measurement scales in 2D images (including astronomical pictures)

Let us begin with some recent astronomical images, and then take up the issue of measurement scales in those images. See Phil Plait’s good collection of the best astronomical images of 2006
here.

Few of the images have a scale of measurement built into the beautiful picture, yet the sizes
are deeply interesting.

Go to the original sources cited by Plait for helpful details and explanations.