\textsc{png} is the preferred input format because
\begin{itemize}
\item	\textsc{png} uses losless compression and a good compression ratio
and
\item	there is a library (\textsc{png} reference library) capable
to open/read all \textsc{png} images.
\end{itemize}

The \textsc{png} file format is a good choice for ``artificial''
images, i.e. logos, screenshots\ldots