Change thumbnail for chart template

I have created some customised chart templates that are saved
as .crtx
files. When I unzip these files I see a whole bunch of xml files,
folders
and ... in a folder 'media' I see the thumbnail image1.bmp that is
displayed in XLS and PPT.

I though this might be a way I could make my
customised charts stand out so I have tried to substitute a new
image1.bmp. and then rezipped all the files and folders and changed
the extension
back to .crtx

Unfortunately I must have done something wrong as when I go back to
the 'My Templates' window the .crtx file and the thumbnail is
missing.

Is there a file containing a list of .crtx files somewhere else that
is used to determine what is shown?

I have tried posting this on
http://groups.google.com/group/micr...cfd61639c9
but without luck.

Any one have an idea?

Thanks

Steve
XP & PPT'07