You can't split test organic pins properly, so stop trying to make it clean and make it honest instead.
I change one thing. Same URL, same board, same week, two designs, a day apart. Then I leave them alone for a month, because a two day read on a platform where pins take weeks to find an audience tells you nothing at all.
The hard part is not touching the headline and the image in the same test. I did that for a year and learned precisely nothing from it.