The AutoCAD Align command can move, rotate and scale an object while aligning it with existing geometry or selected point.
It’s often only thought of as a 3D command but is one I often use in 2D. While it will manipulate AutoCAD geometry you can also scale and align images. Just draw a line to the “known length”, select the two corresponding points on the image and use Align with the scale option to rotate, scale and place the image perfectly.
Here is a Camtasia gif of Align in 2D action.
I’ve long argued Align deserves a place on the standard “Modify” toolbar/Dashboard but it’s not there.
UPDATE 2010-06-18: AutoCAD 2010/2011 have Align on the Home > Modify flyout
Older AutoCAD versions may still require you to add it:
Thankfully the ability to customise the dashboard means you can add it yourself using Right Click > Customise Commands and choosing “Align” in all commands. If the control lacks an image, specify RCDATA_16_ALIGN in both “Images” property fields.