The UK government
says:
Anonymous or pseudonymous works (Works of unknown authorship)
In the case of anonymous or pseudonymous works, the use of a work is allowed where it is not possible to ascertain the identity of the author. It must be reasonable to assume that copyright has expired, or the author died over 70 years ago.
So how likely is it that the artist (whoever they were) died more than 70 years ago?
I'd recommend leaving them out. Sorry.