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Thanks in advance for the help. Hope it made sense. :)
Look for the piece on "Conditional Expressions"
Apache Tutorial: Introduction to Server Side Includes [httpd.apache.org]
Apache Module mod_include [httpd.apache.org]
On the second page take a look at "Include Variables", specifically DOCUMENT_URI, and "Variable Substitution". There are some examples there for using the DOCUMENT_URI to test for the location of the file. You can test for directories as well but I can't quite remember the proper syntax.
Then you could have a conditional statement in your nav bar that figures out what page it is being included by and shows the appropriate images. Remember that DOCUMENT_URI will give the path of the page doing the including not the page being included.
Either.or is fine for these types of things. If you start getting into more complex conditonal statements and doing more things I would just go with php.