At the time of writing the miniltx did not reliably perform driver detection with TexpadTex.
It is straightforward to fix this by forcing the driver when loading graphicx.zty.
An example of how to do so is show below
\input miniltx
\makeatletter
\def\Gin@driver{texpadtex.def}
\input graphicx.sty
\makeatother
Before image
\includegraphics[width=5cm]{im}
After image
\bye