This picture in fact a collage of two illustrations by Ciruelo Cabral.
(ed.CONS: pictures added. Please use report feature to submit inspiration picture links, thanks!)

I'm convinced this is a paint over. Although the image itself isn't mind blowing, I don't believe anyone can copy the marble texture this precisely, even with a grid.

It's reasonable for a similar texture to be painted to give the same impression, but every little spot and detail is exact.