X3 already adds a "download" button for the original image you uploaded.
Or you want download links to different, even larger source images than the ones you uploaded? That can be achieved with dynamic links. If would require that you upload your original sources of course. See tutorial, and specifically see the "Download high-resolution images" section: viewtopic.php?f=61&t=9235
That would require a PHP script that ZIP's all images on the page in real-time (on user-click), and serves the ZIP to the visitor for download. It would then destroy the temporary zip file and repeat the process for any visitor that clicks to download the zip.
It can definitely be done, although there are some caveats: 1. Unless you have a powerful server, it may timeout if attempting to zip too many images at once. 2. It kinda exposes all your gallery folders for easy download.
It's been requested before, and definitely something we are considering to implement. I will bump it in my todo list.