I finally tracked down the artist for the reference I used 4 years ago for a practice snow piece in acrylic. I found it today by putting a photo of my work in Google. Then contacted the artist on Instagram and had a lovely chat with him.
I explained that I had originally found his work on Pinterest which was posted several times without any acknowledgement of the source.
It’s so important to get permission when you can, whether or not you intend to sell the artwork. It’s been bugging me for years because I don’t like to use references when I don’t know the provenance. I know there are several free sources available but this one appealed because of the composition and palette, though I shifted the latter.
The artist ( @sergeioussik on Instagram) sent me his original 2006 painting (Bottom photo) which is in pastels.
For interest @ajalper on Instagram did a recent post on the minefield of copyright, etiquette and permissions. Worth a look.