In XL 2003: View | Header & Footers : Custom Header; use the Insert Picture
tool - it is the second from the right on the Header dialog
Similar in XL 2007
Details at How to insert a picture into the header or the footer in Excel http://support.microsoft.com/kb/826918
I believe the XL 2003 method works in XL 2002 (too long ago to recall with
certainly)
For ealier Excel version http://exceltip.com/st/Adding_a_Picture_to_a_Header/Footer/202.html
best wishes
