I am receiving the following error message:
2011-02-11 14:47:13.815 myProject[13177:207] Could not load the \"icon-troubleshoot.png\" image referenced from a
as far as I know the search tool of x-code do not search inside the xib files that's why your search returns nothing. Anyway It's really probable there's still a reference in a xib file somewhere. Because xib files are just xml, if you don't want to check all them manually, try to open all your xib with a text editor like TextMate and perform a global text search over the content for the .png filename.
Hope this helps. Ciao!