Change with on Blueline template

I am trying to change the width of the “coin slider” image (header? the wider blue stripe with the rotating images) on our homepage:

https://www.partnersinsexeducation.org

I’ve changed the image, and the width of the image, in header.php – but I can’t find the command that sets the maximum width for this image

(I’m new at this and somewhat self-taught – I’ve searched all of the css’s for “max-width” with no luck)

Any help is hugely appreciated