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.
gthumb play videos if you installed proper codecs
Will check it out!