I found mine in
Library/Caches/
with the name
com.appleuser501pictureCache.tiff
If it still exists you can find it with EasyFind. Download it for free from here:
http://www.devontechnologies.com/products/freeware.html
In EasyFind's searchbar, type
*501picture*
Select 'All Volumes' if you want it to search TM too.
In the left panel, check these options
Only Files
Unix-wildcards
Ignore Case
Invisible Files
If that doesn't find it, try just searching for
*.tiff
and sort it by 'Where' in the columns so you can rule out all the obvious ones at a glance. You can use 'Quicklook' in EasyFind too. Just click on any entry you want to check and hit spacebar.
EDIT: BTW, I'm assuming you took this picture when you did your set up. If you took it later it could be in Photo Booth; check out
~/Pictures/Photo Booth Library
Control click on Photo Booth Library and choose 'Show Package Contents', then navigate to 'Originals'.
Message was edited by: softwater