...
my $dir_to_open = "../../pics";
my $dir_to = "../../back";
while (my $row = $sth->fetchrow_hashref()) {
    for my $imgfile (values %$row) {
        if (-f "$dir_to_open/$imgfile") {
            rename "$dir_to_open/$imgfile", "$dir_to/$imgfile";
        }
    }
}