https://www.gavick.com/joomla-templates/quark/install-restaurant#galleryIt is explained here.
And some extra information i made myself:
My way is to :
1. First choose the 15 pictures
2. Rename the pictures to image1 image2 image3 etc.
3. Make thumbs with the filename thumb1 thumb2 thumb3 etc.
4. Copy & paste in filezilla in images/demo/gallery/
5. Change the resolution of image1 image2 image3 so it match.
This way you do not need change the file name in the module but only the resolution.
This the way i do it. It takes around 10 minutes for 15 pictures.
It is still a really nice gallery but it takes time to set it up
If you need help feel free to ask