If a post has no artist but has the source, why not put the source artist's name as the artist? The only flaws I see is the requirement to find said name, and find said art to prove it to be the source.
In addendum, if said image has more than one artist, or has more than name for the singular artist (A-Z as artists, A/B/C... for more than one name), just use the one found. This would "create" more artists, several possibly the same, but'd give them identity. I have not done this yet, in case there is a rule against it, but i would when i see it (and am not busy).
Updated by CrocoGator