I misunderstood the question originally, but leaving this answer in case it's useful for others in the future. For reference, I misunderstood the problem statement to be like:
I'm worried that if I create Header/Footer images from local file paths, that those images won't render when I distribute the file to someone else.
This is not how Header/Footer data works, here's a lengthy explanation.
I tested by adding a picture from a file path, and then deleting the picture from that location ("c:\debug\Leftfooter.png"
). In other words, no file exists at the original file path. This should be essentially the same case as a co-worker with different drive mappings or permissions, etc.
Upon re-opening the file, the footer image is still visible to me, even though it no longer exists on my hard drive:

If the file no longer exists, how does the image persist in Excel Footer?
Now, the image file exists within the XLSX's .ZIP archive as xl\media\picture1.png
. So the file has been incorporated as part of the Excel Workbook, and not as a reference to an external file object.

I observe that upon querying Sheet1.PageSetup.LeftFooterPicture.Filename
, the return value is now simply "Leftfooter" (the original filename, without the path qualifier).
And digging a little deeper in to the underlying xml, the relationship appears to be mapped within the xl\drawings\_rels\vmlDrawing1.vml.rels
which is connected to the xl\worksheets\sheet1.xml
by the `"rID1" Relationship Id.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
<Relationship Id="rId1"
Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/image"
Target="../media/image1.png"/>