first add the image that you want to be clicked on,
now click on the image: you will see a box around it
like this:
now once you get the box click on the insert link button at the top
a box will open in that box you can link to a page that is on the group; or make it linkable to another page off the group but typing in the URL
example:
now you should have a linkable clickable image
Nip