There isn't really an easy way (as far as I can tell).
You designated a "longdesc" attribute for each image. That's how Galleria knows where to link the bigger versions of the images.
Unfortunately, Galleria doesn't simply insert each <img> in an <a> element ([sarcasm]that would be silly[/sarcasm]). Nor does it wait for a normal "click" event (which, if I remember right, is only fired by left-click). It sends you to the "longdesc" location on mouseup.
Even worse, the function that actually decides whether or not to do this isn't in a normal event listener, so it doesn't have access to the "e" object, so you can't just do something as simple as check e.which!
There are workarounds, but they get exponentially more complicated and infuriating the deeper into it you go...
In retrospect, this post probably isn't that much help. To OP, or anyone else interested in pursuing this: The madness begins at about line 3,750 of galleria-1.2.6.js; that's where it decides whether or not to open the link.