If the script came with a .css file (it can be style.css or gallery.css etc.) you can change those values in there.Or as I am seeing there are two values [$breedte = 250;// width van de plaatjes / $hoogte = 250;// height van de plaatjes].I am not sure what language it is, but I'm guessing those are the functions or values to set images height and width.Default as I'm seeing are 250x250.You can try and change those and see what happens.
And for the popup you can use javascript.There are lot's of them.http://www.google.lt...iw=1920&bih=946