Any way to specifiy the 'target' of thie link, so that it stays on the same window (and doesn't open a new one)?
90 seconds later: I found it. There is a property called imageOpenNewWindow. I couldn't see it at first because it was truncated on my display. Just set this to False, and the new LP will be displayed in the same window.
Select your image or item you want to add a link. Go to the Property Sheet, that's on the right side of the page, left of the Landing Page Actions, Where it says linkURL, paste or type the url. That will add a link to your button.