:-) Nice! Job
Quit Chrome (ensure you don't have any running)
Find the Chrome application directory (e.g., on windows, it is %USERPROFILE%\AppData\Local\Google\Chrome\User Data
Put your new avatar png file in the "Default" subdirectory.
Edit "Local State" in a good text editor
Find the "profile" section; it resembles this:
Ignore the "avatar_icon" line - it's not what you want!
Edit the "Default" profile section and add the following lines (I put them in the right alphabetical order amongst the other lines; I'm not sure whether that matters):
Save the "Local State" file.
If you want to do this for other profiles, just do the above steps for a different directory besides "Default" (for example, "Profile 1").
I had run chrome with the --gaia-profile-info switch before I tried the above, so you may need to do that first, but I'm not sure.
You may want to star the Chrome issue 91230 http://code.google.com/p/chromium/issues/detail?id=91230 to get a proper UI for this type of thing again.
I love it!