It varies, I think -- probably the majority of Japanese fansites take measures to avoid getting listed on normal search engines, but a few slip through the cracks here and there, and art thieves might find them that way. Also, most fandoms of any decent size have fandom-specific search engines where participants will list their sites so people inside the fandom can find each other. Once you've managed to find one of these for a given fandom, you're pretty much set, so I imagine many people looking for art use them as well.