A small thumbnail is a tiny image or picture that represents a larger image or video. It's usually displayed as a small preview so that users can have a quick idea of what the full-sized content looks like. Full definition
The 2000AD reader adds a scrub bar along the bottom, showing small thumbnails of each page allowing you to run your fingers along through and entire issue, a surprisingly fun thing to do. (cbr.com)
It basically slurps up your Instagram feed and puts the most recent images as small thumbnail images, which show up when you swipe down from the top of your iPhone or iPad. (cultofmac.com)
(Below the large image is a run of smaller thumbnail images that can be scrolled; clicking on one of these will, eventually, cause it to appear in the upper viewer, from where «zoom view» takes you to a clear and expandable view.) (slaw.ca)