I don't have Tiger handy, so I can't test this. What I would do is:
1) Load a gallery page in Firefox 3.0 in Tiger
2) View Page Source, or Save As Web Page, HTML Only
3) Load same page in Firefox 3.0 in Leopard
4) View Page Source, or Save As Web Page, HTML Only
5) Compare the source code from both views. BBEdit can do this, as can the 'diff' command in Terminal
One can also right-click on the (missing) image to see its source. Gallery does some magic since images aren't usually hosted in a web-accessible directory.
Another consideration is if URL-Rewrite is enabled. Is it? And if so, which method? .htaccess (mod_rewrite) or the PHP rewrite?