My new favorite way to find images for facebook and blog posts is to search the Google Arts & Culture site. (Click the magnifying glass at the top right of this page)
I follow the external link and find the painting at the YCBA site.
Which says it's in the public domain, but you should credit it to "Yale Center for British Art, Paul Mellon Collection", so it's fair game to use in postings or webpages.
For extra credit, I use their IIIF link and my IIIF judo to crop and resize to just the part of the image with the magnifying glass in the size I want to use.
And I have a beautiful eye catching piece of art to illustrate my post with!