[xls] Image export causes warning with XL

Submitted by Frédéric Parrenin `@parrenin`

Link to original bug (#757586)

Description

Created attachment 314812 .xls file to reproduce the problem

Steps to reproduce:

open the attached .xls file in gnumeric
make a trivial change
re-save the file
open the file in XL (tested with XL2007) => A warning appears, and the image which was in cell D19 has disappeared

Attachment 314812, ".xls file to reproduce the problem":
non-employee_expenses_form-FP.xls

Version: git master