Can't find background max-width.

Hello,

This afternoon I downloaded a background-plugin (which did not work because of old php), but even after uninstalling, my background-image has no max-width. For now, I can not find the place where I have to give a max-width to my background image. (website: https://www.fransvandervleuten.nl)

I already checked with fire-bug, but that’s not helping my.

Who can help me find the place to change my background image max-with?

Thanks!