Cannot remove background photo on one specific post

Hi,
At my website every page has a white background (left and right of the header/content/footer)
https://prperry.nl/

I do not want a colored background or some photo.
Why is this post having a background? https://prperry.nl/wat-is-contentmarketing-een-definitie/
I mean the purple photo. I didn’t add it and I cannot delete it.
Where can I change the settings?
THX

PROBLEM SOLVED.