HyperLink HyperLink1 = (HyperLink)Master.FindControl("HyperLink1");
if (HyperLink1 != null)
{
HyperLink1.NavigateUrl = "http://www.google.com/";
HyperLink1.Text = "Changing Text From Child Page";
}
This code will find the hyperlink present in the master page and set the text of the hyperlink.
Similarly you can add change the image,which are present in the master page .