Centering with \includegraphics not with \begin{figure}?

Put \includegraphics into a center environment -- this also adds some space before and after your picture.

\documentclass{article}

\usepackage[demo]{graphicx}% "demo" to make example compilable without .png-file

\usepackage{lipsum}

\begin{document}

\lipsum[1]

\begin{center}
\includegraphics[width=0.5\textwidth]{mypicture.png}
\end{center}

\lipsum[1]

\end{document}

\includegraphics{..} looks to TeX like a big letter, center it the same way

\begin{center}
  \includegraphics{...}
\end{center}

or use \centering if you prefer.