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\titlegraphic{
\includegraphics[width=2cm]{TeX.png}
}
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% Multiple logos
\titlegraphic{
\includegraphics[width=2cm]{TeX.png}
\hspace{2cm}
\includegraphics[width=2cm]{TeX.png}
\hspace{2cm}
\includegraphics[width=2cm]{TeX.png}
}