Board Thread:Questions and Answers/@comment-31617896-20170331063725/@comment-6380028-20170331064958

There's really no set rules. It's really just what look best and the images available. Sometimes good transparent images that just very rare so they use other images, like the ones in Tracer VS Scout. I'm not too use why they didn't use a outline for that Zoro VS Erza thumbnail, but my best guess is that the Zoro image already has a black outline so it will just look distracting to have a white outline on top of that. Either that or they just didn't bother to.