Is there a way to generate the cache offline? Lets say, the gallery has several albums with several photos. Do I have to view each single albums and each single photos in order to generate its thumbnail in the cache? An script generating the thumbnails offline might be nice.

You can definitely make such a script, just as a php page that loads each image in the gallery.

If you modify i.php, you could add a flag that makes it not load the actual image in the browser so that's faster.

Other options are to use a batch image resizing program, and use the naming scheme that ZP uses. They're named like:

[folder name][file name][size].jpg
ex: Desktop Pictures_Dog and Cat.jpg_480.jpg

Then pop them in the cache/ folder.