On one website, I saw that the webmaster somehow had a random string of numbers at the end of the image's url. The numbers always changed everytime the image was reloaded, and this prevented the image from being cached. But I don't know how to apply that to what I want to do. They must've used javascript or something to make the numbers change.
Anyway, does anyone know how to force the image to reload everytime the page is visited or refreshed?