how to define a background image for new tabs

Are you asking on how to have a different background, or wallpaper for each new tab, or just the same image for every tab?
I do not know of any way to give a different image for each individual tab or window, but there is a way to change so all tabs show a selected image.

It involves creating a CSS file that Firefox will load on startup, and adding, or changing, certain entries in your profile and in the config.

You can get the details in this outside article on the superuser website:
https://superuser.com/questions/1495946/how-do-i-change-the-background-image-of-home-page-in-firefox

NOTE: This link will take you outside the Mozilla Support site. You will not be required to register, or submit any information.

Some New Tab Add-ons allow you to import a file, such as

https://addons.mozilla.org/firefox/addon/new-tab-tools/

However, if you prefer to keep the page just as it is and inject an image into it, the first reply describes what you need to do. See also: https://support.mozilla.org/questions/1293694