The lack of an option to swap vertical and horizontal resolution on this site annoyed me, so I made a boorkmarklet to do it for me (I don't really like userscripts that run automatically, though this could easily be adapted to one). Might as well share. Only tested with Firefox.

Make a bookmark with the following "URL" and select it when the generator is open and "custom" resolution is selected. "Width" and "Height" will swap values:

javascript:{let [w,h]=['Width','Height'].map(n=>[...document.querySelectorAll('label')].find(e=>e.textContent==n).parentNode.querySelector('input'));[w.value,h.value]=[h.value,w.value];[w,h].forEach(e=>e.dispatchEvent(new Event('input')))}

Note: like all bookmarklets, this is liable to break if the site ever changes. But right now it works.