man pages don't have screenshots. They typically use a pager, but they can have formatting. Just type 'man man' in the cli without the single quotes. Online used to mean documentation provided on the local system.
Of course, there is not a reason why the manual page cannot have images and figures. The whole PIC language exists to put diagrams into things like ?roff, and it was at one time de rigueur to format the manual pages to postscript and send them to the printer. It was also common in later times to generate PS or DVI output and view that with the appropriate viewer. Honestly viewing the manual pages with a plain text pager is the new, worse-is-better method.
Most man pages give good PostScript output with "man -t". I like reading those on screen or occasionally printing manuals I refer to frequently. The main problem I've run into in recent years are man pages that use ASCII art but don't wrap that in explicit roff requests for a fixed-width font, making the result mostly unreadable outside the terminal.
I guess that if we manage to force sixel support into xterm, QTermWidget, and VTE, we'd have a reasonable excuse to add graphics support into man and info for terminals that have sixel graphics in their termcap entries.