Unknown background image when using WPSupersized

Hi there,
I’ve noticed that my site has an unknown background image which is visible under the footer of my homepage and also for a split second when you view any other pages of my site: http://colourpopdesign.com.

I have set the About and Contact page to have a background image but this image seems to be behind those images. It also links to a site I do not know. I have checked the box for supersized to not work on my homepage but it is there too.

Can someone suggest how I could get rid of this image please? Thanks!

https://ww.wp.xz.cn/plugins/wp-supersized/