Hi Don, we were posting at the same time, so you may not have seen my 2nd reply above.

Check out this animation for a demo of how to position and scale an existing image to match the size of a horizontal line by first dragging it, and then using the scale grip:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2501.15

That one is showing aligning it to another image, but aligning to a drawn line is the same.

Note that the tricky part is that when you go to scale it you need to grab the upper-right scaling grip and then drag that one to the end of the line. You want to grab that one because you want the bottom-left corner to stay fixed at the start of the line where you just dragged it, and when you drag from the corner the scale origin is fixed to the opposite corner from where you are dragging from (unless you click and release on a corner grip instead of dragging on it, which toggles the scaling mode between "scale from opposite corner" to "scale from center" modes).

Here's another video showing a similar kind of alignment to an existing rectangle:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1982.6

And a few various methods including the Align tool are shown here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2501.14

- Michael