IrfanView is a great piece of software for Windows. One of those “jack of all trades, master of none”. It works as a simple image & video viewer and browser, can do simple and not-so-simple image manipulation, and even batch jobs, like conversion, on selections of images in folders.
For Linux (and other platforms) there’s XnView MP that offers extremely similar functionalities.
Hmm, will have to check that out… I use ImageMagick (mogrify/convert) on the shell, Gwenview and Krita but not for batch stuff (and pics only, not videos).