"it" is what?
An Excel workbook named invoice.xls?
The default Excel 2003 Sales Invoice.xlt template has no Logo to change.
Versions earlier than 2002 had an Invoice.xlt template that contained a logo and
a method of changing it.
Perhaps you have an invoice based on that version?
Click on File>Properties and see where the file originated and what is stored
under Contents and Custom tabs.
Gord