code:

// ==UserScript==
// @name               sa restrict image height
// @namespace          [url]https://forums.somethingawful.com/[/url]
// @description        asdf
// @include            [url]https://forums.somethingawful.com/*[/url]
// @grant              GM_addStyle
// ==/UserScript==

/* jshint multistr: true */



/********** FUNCTIONS **************/
function handleImageClick(e) {
    var $target = $(e.target);
    var $img = $target.closest('img');
    $img.toggleClass('fullHeight');
}



/************** STYLES **************/
$("body").prepend("	\
<style> \
    video { \
        max-height: 80vh !important; \
    } \
    img { \
        max-height: 90vh !important; \
        object-fit: contain; \
        object-position: left; \
        cursor: pointer; \
    } \
    .timg_container img { \
        max-height: none !important; \
    } \
    img.fullHeight { \
        max-height: none !important; \
    } \
</style> \
");


/********** EVENT LISTENERS ******************/
$('img').on('click', handleImageClick);