Thanks for your reply, acrylian!
I understand that changing that line will take the resized image and display it instead of the original large one.
Unfortunately, it seems I have no resized images... changing that line caused my lightbox (slimbox

to not find the image.
In the album folder, there is only the original (large size) image and no resized image. I only have one image upploaded in that album.
I wonder why my zenphoto doesn't resize the images that I upload...? I have set [i]"Protect full image"[/i] to false and [i]"Full image download"[/i] to false.