Trace (draw a line) over the known-length line on the picture, then divide the known length by the length of the line you've drawn. You can find that in the selection info palette when you've selected the line. Then select the picture and tab into and through the inspector bar x and y scale boxes, entering the scale factor you worked out, then hit enter. Selecting picture and line and scaling them together will verify your working, as the line should now be the known length. The selector point hub is the station point, use ctrl + d to position it at one of the ends (v snap) of the line while both picture and line are selected.