mmmkay, how familiar are you with prototype? because that's what you're going to need here i think. let's start with what i think the logic would look like and go from there.
let's say the image that is currently up will have a z-index of 0
you make your double click, pass the incoming img id as an argument
z-index on your incoming img is set to 1, it fades in on top of the outgoing img
find all img elements that are visible and NOT the incoming img (so, the outgoing img)
iterate through the list of img's and fade them out from behind the other image
set the z-index of the incoming img to 0 (so that the next one can repeat the process)
so you already know how to use the effect toggle. what's left? setting z-index
, using prototype to find all visible img elements
, iterating through the array
of visible img elements and telling your function what to do with them.
that should get you started or at the very least give you some ideas.