It should be since this is a PHP site, and any fullscreen links are created by the same script.

Could do the following:
replace an insert of fullscreen.PNG (or $fullscreen as an example variable) in the current code with "/fullscreen.php?fullscreen" or "/fullscreen.php?$fullscreen" with variable

If you want to make it really lean on the existing code:

/fullscreen.php?pic=$fullscreen&?id=$id
("$fullscreen" and "$id" being substituted with the existing variables)
Since id is already used to call details and the download link, the code for the fullscreen page would be v.simple

And, if there is a function that calls fullscreen other than the ones that are known (and used every day), then it can continue to call the PNG itself, no hassle.