Image without extension as Canvas Background

Hi there

I've inherited a work of an ex-employee and have to change the background on one report. 

There are multiple pages (e.g. title page, chapter page) and each of the chapters has same background. Now, I would like to change that background at one place instead of going page per page. Also for the sake of maintenance.

It does not indicate that the Canvas Background is a .jpg file, rather it says it is "Wallpaper". The other pages (title, chapter) are referencing a .jpeg file which is hosted on our Sharepoint.

Could someone tell me if there is a central place to edit this, or what is the reason that it is called "Wallpaper" without having a file extension like the other slides? Thank you in advance.