diplaying Image by specifying path

Hi,

I am calling image in reports by specifying path in formula column.  If image is not available in server in specified location. how to pass default message or default image.

Formula column:

function CF_IMAGEFormula return Char is
  lv_Image  Varchar2(200);
begin
  lv_Image := '/dbapp/db/EBSCRP/ebscrpcomn/java/oracle/apps/media/'||:employee_Number1||'.gif';
  Return(lv_Image);
end;