Hi Slatki,
It could just be that xat does not recognise the domain you have used to upload your background to.
I have reuploaded the background you have to imgur, one that is known to frequently work.
Here is the link to your background on the imgur website: https://imgur.com/a/Ll0hykv
When adding your background to the chat, please use the image's direct link here: https://i.imgur.com/o2xoM6w.png
Hope this works and please reply if the problem persists!