There are a variety of photo editing software programs available, but only a few of them have the capability to measure distances. I will be using Adobe PhotoShop Elements 3.0 - PSE3 - for this supplement. This is the least expensive member of the Photoshop family ($90). I intend to illustrate the useful properties of this program and I do not present this as a tutorial on PSE3.

The operational components are described in Fig. 1.

Fig. 1

I will define the following operations so as to avoid repetition of instructions.

Click. Depress and release the left mouse button.
Drag. Depress and hold down the left mouse button. Move the mouse so as to position the cursor at the desired position and release the left mouse button.

To make your selection of preferences, click on Edit, then click Preferences on the displayed sub menu and then click on General. Make your selections and then click on Next to advance to the next Preference group. For Units and Rulers, I selected mm for Units: Ruler. For Grid, I selected Light Red for Color, 20 mm for Gridline every: and 1 for Subdivisions.