data1.json is generated by jAlbum and contains the list of images for this folder. Minimal doesn't have this file, because that's not a JSON-based skin.
folderimage.jpg is the hero background image. What would the skin use in the hero instead? I know that creating these views with these extra folders will result in numerous, virtually unnecessary files, but how is the skin supposed to function without them?
folderimage@2x.jpg is the high-DPI version of the same image for retina displays. You can turn this off in the
Tiger / Advanced / Site admin / High resolution theme image.
folderimg.jpg and
folderimg@2x.jpg are the mobile-optimized (vertically cropped) versions of the hero background.
folderthumb.jpg is the folder thumbnail. Today, we have a separate setting for the folder thumbnail size, but Tiger has generated these images far before jAlbum offered it.
index.html - I don't think this will bother anyone.
shareimage.jpg is the folder's share image, matching Facebook's requirements.