$jsonString = '{"body":[
{"name":"Background","color":2,"img":""},
{"name":"Background","color":0,"img":""},
{"name":"Background","color":3,"img":""},
{"name":"Background","color":1,"img":""}
]}';

$response = json_decode( $jsonString );

usort( $response->body, function( $x, $y ) {
    if ( $x == $y ) {
        return 0;
    }
    return ( $x > $y ) ? -1 : 1;
} );

echo $response->body[0]->img;