let me explain...
for example in VB, there is a "session limit" function on admin panel
this is a limit for online users
if you set this to 20 and your vb has 20 online user, then 21th user can not enter the site
while attacking, too many proxies use for multiple registeration and so too many users are online and this force the server and the site'll be down.
so if we setup a session limit for the script, there'll be no problem (with image verification)