Starting in the email editor, click on the panel to make it active.
From here:

• Click on the link icon over any image;
• Choose your link type, e.g: Web Address;
• Complete your selection, e.g: for web address, type in the full address (URL) of the site you would like to link to.
• Click Apply

If you have more than one image in the panel, each image will have a separate link icon.