We use Selenium to automatically capture the screenshots. It seems that Selenium is unable to capture your website correctly on the iOS simulators.
Unfortunately, there are no immediately available solutions as this is the default behavior of Selenium on the iOS simulators. You’ll be able to test other URLs and devices, just fine.
Also, we are working on a Real Mobile cloud, where we will have actual iOS, Android, etc. physical devices hosted for testing and we expect such problems to be resolved there. We’ll be sure to keep you posted on developments.
Keep an eye on the browserstack website, more good stuff is on the way!