Hi Seb,
That's true that Artisteer header sliders are not fully responsive.
You can use the workaround provided by Tanya in the forum thread mentioned in the second message or use this code:
.responsive .art-slide-item {
background-size: 100% 100%!important;
}
You can play with the % values to adjust the look of your slideshow.
Best regards,
Elvira