I could use any image (iso) file I find as long as I use my CD-key right ?
I can no longer use the external DVD-ROM cause I'm at college again.
Not necessarily, because in there has been a change in regards of the CD-key system. Nowadays, IIRC, CDs require a 24 character key, whereas before, they only required 16 characters. (Exact numbers might be different but you get the idea.) If you are unlucky and get an installer with a different code requirement than your key, then you'll be unable to install.
Blizz solved this by converting your string into the new system, once you added your key on your bnet profile.
I say you stress test your mind and try to think about that registration, if you really wanna play, or buying the game online with a new acc straight from blizz. (May be a bit costy, tho.)
