If you want to change your X Windows background, feh will need to know the name of the X Windows display to use. It will expect that name to be in the DISPLAY environment variable. If you only have one display on your local system, its value will be ":0.0" so you could use this:

system("DISPLAY=:0.0 /usr/bin/feh --bg-center /home/user/Pictures/rota
+te/1.jpg");
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or better

$ENV{DISPLAY} = ":0.0";
system("/usr/bin/feh --bg-center /home/user/Pictures/rotate/1.jpg");
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