jxh2154 said:
I'd think that using pixiv as the source URL is a bad idea if you have to sign up for the site. At the very least, when it's just as easy to get it from the artist's site, that seems better.

I usually put the pixiv link as "extra URL" if I have a proper homepage for the artist.

Perhaps a "Pixiv number" section on the artist form would be a good idea?