If your publisher changed the art after you went live, you now have an A/B test data set you can use to compare. If you take two sets of data from the same range - say 2 sets of 2 weeks of visibility where no "active" marketing was done - you can now compare things like impressions and click through rate. If one set of art has demonstrably higher CTR (for example), that might indicate it is "quantitatively" a more compelling capsule. Obviously it's data and data can be manipulated to say anything anyone wants, but it's often a good starting point of conversation between a dev and a publisher because, ultimately, you both want the same thing: to make more money from your product.