Need advice on an avatar problem

[TinCow1]

I currently run a site with 4.2.0. I took over from the former Admin after died unexpectedly about a year and a half ago. The former Admin did all kinds of custom changes to codes and never documented his work. One of the things he did was to install a default avatar system. At the time I took over, this plugin was installed, but it was non-operational and changing settings with that system did not change the avatar that was getting assigned by default. I worked around this for a long time by just uploading a new image in the same location and with the same name as the one which was being pulled. However, I'd like to fix this issue once and for all now, and just revert vB back to its default behavior on avatars for new registrations.

The avatar that gets pulled as the default is named "8255.gif". I'm having difficulty finding whatever code it is that references that file name or which directs to that location. The default avatar plugin linked above is uninstalled and thus is not responsible. What templates and php files should I be looking in to check for custom coding which may be causing this?