To select points from gravitationally lensed images, we'll start from the
file we saved in the previous step. The basic idea is to add a 'points layer'
for each lensed image you want to use. So, we'll add such a layer and select
the 'add point' tool to be able to select specific points as input. You
can change the size of the circle which is used to mark the point you selected
either in the layer settings, or in all layers by selecting 'set point size'
in the 'edit' menu.

WARNING: make sure the layer you want to add points to is marked as active!
In this example, I actually forget to do this at a certain point, resulting
in only four point layers for five lensed images. When disabling/enabling each
layer in turn, you can see that the points of the leftmost image and the 
center image have been put in the same layer. Luckily, by selecting points,
copying them to another layer, and deleting the original points, it is not too
difficult to overcome this problem.

When exporting the image points, the currently visible point layers are stored
into an images data file. Each layer is interpreted as a single image in the
gravitational lens system. All the images of a single source should be stored
in the same images data file.

Some inversion methods can take matching points in different images into 
account. To indicate which points are lensed image points of the same source 
point, you can assign them a label using the 'group number' point.