Comparing my XML code with other examples, I found that the MS Office
2007 Custom UI Editor apparently assembled my image relationships in
the "customUI.xml.rels" file out of order. Instead of the apparently
correct sequence of:

<Relationship Id="myID" Type="http://schemas.openxmlformats.org/
officeDocument/2006/relationships/image" Target="images/myIcon.png" /


.... it assembled the string in this fashion (with the Relationship Id
at the end instead of at the beginning):

<Type="http://schemas.openxmlformats.org/officeDocument/2006/
relationships/image" Target="images/myIcon.png" Relationship
Id="myID" /

Since this latter code was built entirely be the MS Office 2007 Custom
UI Editor, I can only conclude that this is a bug in the editor,
possibly activated by my building my Ribbon button XML code first,
then saving the *.xlam file, and in a subsequent session, adding the
icon.png files.

Perhaps this will help someone else.