I was either too young (in the time before the prequels came out) to have bothered to imagine what Anakin would look like or remember what I had imagined.
I guess I probably imagined Anakin as the Sebastian Shaw ghost we see. But, at the same time, I knew he must have been a lot younger when he became Vader so he never actually looked like that (why does he look like that? it's dumb, but of course that's a different conversation). Also I've never known what young Sebastian Shaw looked like, and I've never bothered to Google it. Who is that dude anyway? I've seen a lot of old movies and he was in none of them.
It's interesting that so many pictured him with dark hair. Is that because he turned bad? What are you saying about people with dark hair? As someone with pretty dark hair myself, I have to say I'm offended.
I never had a problem with Hayden as Anakin, looks-wise (though I think showing a child age version of him might have been Lucas's biggest mistake). That acting though...