I was trying to keep SPGM on drive C: under htdocs and keep only the galleryies under D:/Pictures.

It was easier to just install SPGM under D:Pictures and allow Apache to have access. This is done by:

In the <IfModule alias_module> section add the line:
Alias /Pictures D:/Pictures

Then add a Directory section as follows:
<Directory D:/Pictures>
AllowOverride None
Options None
Order allow,deny
Allow from all
</Directory>

Now everything for SPGM resides there and all works as expected.