hi... why does this work (btnContinue is an ImageButton)?

btnContinue.Attributes.Add("OnMouseOver", "this.src='/site/images/start-over.png'");
btnContinue.Attributes.Add("OnMouseOut", "this.src='/site/images/start-up.png'");

and this doesn't (HyperLink):
HyperLink1.Attributes.Add("OnMouseOver", "this.src='/site/images/print-over.png'");
HyperLink1.Attributes.Add("OnMouseOut", "this.src='/site/images/print-over.png'");

how can i change the image using HyperLink webcontrol?
i need some help...
i don't know what property i have to use so it can work...

thanks in advance...