Is there anything wrong including this png?

Question

I'm writing a document (article class), and I'm including some png graphics, this is the code that includes them (it's a subimage environment):

\begin{center}
\begin{figure}
  \centering
  \begin{subfigure}[b]{0.3\textwidth}
                \includegraphics[natwidth=512px,natheight=511px,width=4cm,height=4cm]{param.png}
                \caption{img1}
                \label{img1}
        \end{subfigure}%
        ~ %add desired spacing between images, e. g. ~, \quad, \qquad, \hfill etc.
          %(or a blank line to force the subfigure onto a new line)
        \begin{subfigure}[b]{0.3\textwidth}
                \includegraphics[natwidth=512px,natheight=511px,width=4cm,height=4cm]{crtpnt.png}
                \caption{img2}
                \label{fig:img2}
        \end{subfigure}
        ~ %add desired spacing between images, e. g. ~, \quad, \qquad, \hfill etc.
          %(or a blank line to force the subfigure onto a new line)
        \begin{subfigure}[b]{0.3\textwidth}
                \includegraphics[natwidth=512px,natheight=511px,width=4cm,height=4cm]{ferro.png}
                \caption{img3}
                \label{fig:img3}
        \end{subfigure}
\caption[prueba]{\label{fig:images}caption}\end{figure}\end{center}

I'm compiling through latex and then dvipdf to get a pdf. Firt of all, if I compile with pdflatex it works perfectly, but I would like to compile with an intermediate dvi file. When running Latex the dvi file is alright, although images don't show up. If I run then the dvipdf, it cuts the document up to the page in which the previous snippet is, not showing the images, or anything else beyond them.

I have previous images in eps format which are perfectly included. Is the way I'm including png graphics right? Am I doing anything wrong?