I meant to post this yesterday, on 3 February 2022...
On Monday, 3 February 1958, in Northampton, Massachusetts, Sylvia Plath wrote at the end of a journal entry that after "Arvin, art & the sudden surprise - Böcklin's 'Island of the Dead'" (322).
Böcklin is Swiss symbolist painter Arnold Böcklin (1827-1901). And his "Isle of the Dead", done in several different versions, dates to 1880 to 1901.
Not sure which version Plath saw, but this below is one of the darker versions.