When you come across a page with too large images, you can use the so-called
Bookmarklets.
Current browsers no longer allow inserting javascript directly into the address bar, but it is possible to bypass this by saving the javascript to Favorites, which then looks like this in the Firefox browser:

Then write the desired javascript in the URL field, for example to shrink all images larger than 800px to 800px:
javascript:var%20i,L=document.images.length;for(i=0;i<L;++i){document.images[i].style.maxWidth='800px'};false();Then, when you come across a page that you wish to reduce the images on, just click on that Favorite.