@jellefaber - That's interesting. I'm assuming it has something to do with filemtime, but we're looking into it. You can track that issue on
GitHub.
@DESIGNfromWITHIN - The main reason we went with screen size instead of window size was because window size is so fluid. If someone starts with a small browser window and expands it to a larger size, we didn't want them to have to reload the page just to see the proper images. If you'd like to switch to something other than screen.width, you can modify the assets/components/phpthumbsup/js/responsive.js file pretty easily (just replace screen.width with something like document.body.clientWidth).
Using screen.width does make things a bit harder to test, but one thing you can do is edit the value of your phptu_width cookie using a tool like Firebug for Firefox. The cookie will change it's value on every page load to your actual screen width by default, but if you change it and reload, you should get images at the right size until you reload again.