For the body ( the screen):
background-image: url("url here ");
For just a element its
background-image: url("url here");
To just color the background i think the command is
No open this link
and upload your image u can get link after upload .dont paste this link in your code u can get image link from here
see this i uploaded one image and after uploading u will get
https://www.sololearn.com/post/277146/?ref=app and copy url link
You cannot upload your gallery photo . You need image link . If u use notepad or any other editor where your file store copy one image and paste where your file is available and click on image and see properties of image u can get path of image and paste in html code with extension of your image .
If u want to upload your gallery image use dropbox u can get image link and paste in your html code
You can add background image using css.
background-image: url( url here);
and you can use background attribute