This is a bug that seems to affect some computers with radeon cards.
https://trisquel.info/en/forum/why-user-and-password-live-cdThere should be a way around this without using a different livecd (and a different livecd probably would not help). Try pressing F6 at the startup window (where you see the options to try live or install, etc) and enable the option "nomodeset". You will not need to enable this though if you want to install it.
Hope this helps!