So the code should be like this:
[dsid="YourMobileContainerName"] {
background-image: url("web_resources/files/resourses/views/assets/image/myImage.jpg");
background-repeat: no-repeat;
background-size: cover; /* comment this out if required */
}
You can use full path as well as shorten.
Ok.. But it still doesn't work.
But now I made a CSS-file (clicked CREATE NEW - CSS). I pasted that code in to the css-file and changed the containerName and the image name. And then what? Do I have to do something in the page-properties to?
Ooohhh!!! I found it out!!!
I forgot the ".." before the path 
Thanks for the help
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How to load image in media manager ??
Add image component on page - go to Properties - Asset - Change- Media Manager will be opened, so you can upload images there.
You can delete component if you don't need it.
thanks
worked for me. thks