The Main Image shows a picture of the current image being morphed. If you have loaded a plot file, the shot lines and will be superimposed on top.
The purpose of the display is to compare the differences between the cave survey data and the sketch-map. This allows you to locate and specify where the errors in the sketch map are.
To do this, you scale and position the cave survey data so the survey stations are close the correct position. When you have done this, some of the stations in the survey data will not match the sketch-map. This is a clear indication of errors in the sketch-map. You tell the program where the errors are by moving the stations in the survey to the corresponding location on the image. This tells the program where the errors are so it can warp that part of the sketch back to its proper place.