The write up. Thanks you.
I found that some time ago :
0 create/find a picture and put it on your Max
1 open the folder where the picture located.
2 click the picture last more than a 2s .
3 choose “set as screen saver”
4 select the sequence and click “ok”
You need to cut your picture to exact size 1200 (W) by 1600 (H). (Max2)
You can probably copy any picture with the proper dimensions directly in the folder that contains the background pictures. You just need after copying to rename the file with the proper name. But I can’t find again the folder path/name.