To link an image, you use the image icon rather than the link icon.
Select the image in the text block (click it so the "handles" appear).
Then click the "Image" icon.
At the bottom of the Image Properties screen you will see an "Edit" button next to the "Link:" heading.
Use the same process to choose the appropriate link that you would for any text-based link. If the link is in the CMS, use the Choose from current subsite, Quick Find (if it's in a different subsite), or unregistered URL if that's the case. Review Link Types.