request: box image different from the wallpaper

[flodus]

Hello I love the work you have done and thank you for sharing!
I use it with batocera on an rpi3 and I was asking myself a question.
In the games lists there is the wallpaper which corresponds to the "image" tag of the gamelist.xml, the video with the "snap" tag, the wheel which I put with the "marquee" tag, but I would like that at the bottom left there is the image of the box with another tag (like "box" or "thumbnail" for example) and therefore different from the "image" tag. Do you know how I could introduce this 4th element in your theme ?
Thanks again !!!
(Google translation)