Hello,
Firstly, in your test case, there are two folders named "files", image is located in the wrong folder.
Secondly, the UniPanel.Background.Fit property is not set True, therefore, you will not see the image, since the size is not small (1920x1200)
Also try this modified code:
with UniPanel1 do
begin
JSInterface.JSCall('setBodyStyle', ['background-image', 'url(files/111.jpg)']);
JSInterface.JSCall('setBodyStyle', ['background-repeat', 'no-repeat']);
JSInterface.JSCall('setBodyStyle', ['background-position', 'center']);
if Background.Fit then
JSInterface.JSCall('setBodyStyle', ['background-size', '100%']);
end;