image not getting refreshed in Qlabel

[prkhr4u]

I am trying to refresh a image in a QLabel.It is getting refreshed when i used this code on a button click:

void Dialog_user::on_btn_Stop_clicked()
{
    flag=0;
    sleep(2);
    ui->label->setPixmap(QPixmap("abc.bmp"));
}

void Dialog_user::on_btn_Stop_clicked()
{
    flag=0;
    sleep(2);
    ui->label->setPixmap(QPixmap("abc.bmp"));
}

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When i am doing the same thing via a signal-slot mechanism,it is not getting refreshed.The code is :

void Dialog_user::ImageRefreshSlot()
{
    qDebug()<<"inside slot";
    sleep(2);
    qDebug()<<"after sleep";
    ui->label->setPixmap(QPixmap("abc.bmp"));
}

void Dialog_user::ImageRefreshSlot()
{
    qDebug()<<"inside slot";
    sleep(2);
    qDebug()<<"after sleep";
    ui->label->setPixmap(QPixmap("abc.bmp"));
}

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I have verified that signal and slot is connected via a debug message. The message "inside slot" and "after sleep" is successfully printed,but the label is not refreshed again.
But again if i am clicking the button,the image is getting refreshed.

P.S.:
I have a image processing application,where the image changes every 2 secs.