3 Aug, 2018
2018/8/3 16:12:32 (GMT) Greenwich Mean Time, London, Dublin, Lisbon, Casablanca, Monrovia
This looks to be a photograph taken by the auction house (Bonham's) to promote what they thought was a good item to advertise. The back says this was for the June 21, 1993 Fine Books and Manuscripts auction and the estimate was £4000-5000.
I cannot find any archive online (press or auction results etc.). I will check my Tolkien Collector back issues to see if it was mentioned at the time.
Should be pretty obvious, this wasn't a painting by J. R. R. Tolkien. I suppose it could be by some random other person with the last name "Tolkien" from 1924.
3 Aug, 2018
2018/8/3 16:41:52 (GMT) Greenwich Mean Time, London, Dublin, Lisbon, Casablanca, Monrovia
Small update - Tolkien Collector #5 (November 1993) mentions the Bonham's auction of the watercolour but does not express a strong opinion on authenticity.
Bonhams, London, offered at auction on 21 June 1993 a large (approx. 10 3/4" x 19 3/4") watercolour painting attributed to J.R.R. Tolkien. The painting is a landscape, a woman and a small child in a field of grain by a cliff overlooking the sea, predominantly brown and blue wash, signed in the corner "R. Tolkien | 1924". The house estimate was £4000-5000. A former curator of an Oxford museum believed the painting to be by J.R.R. Tolkien, and the auction house accepted his view as authentication.