The backstretch script adds a lot of inline styles to its output, which makes it very difficult to override, although you could try tinkering with the CSS on .backstretch-item img
and see if that helps at all (every rule would have to be !important because of the inline styles).
If you have a staging or development site where you can experiment, another option would be to test a new feature I’m considering implementing into the plugin: a scriptless version of the banner featured image. The output will end up being slightly different from the standard/current banner, but is fairly similar, and would be much easier to tinker with the CSS. You can download the feature version from Github. (I would not use on a production site–this is still pretty experimental and I’m not 100% committed to using it, although it’s been promising so far.)