Take a look at this responsive image setup in v10. It might get you want you need from an image size standpoint.
Regarding thumbnails, I believe there is a checkbox in Settings for this to auto generate thumbnails but I believe it's only for media library files.
Regarding media library or page type, if you need granular security on it and navigation options, then go with a page type. If not and you're simply displaying image galleries with minimal info, then use a media library.