
\documentclass[pstricks,border=12pt]{standalone}
\usepackage{graphicx}
\def\Row{5}
\def\Column{5}
\def\FilenameMain{example-image-A}
\def\FilenameChild{example-image-B}
\def\ScaleMain{1}
\def\ScaleChild{0.25}
\newsavebox\IBoxMain
\newsavebox\IBoxChild
\savebox\IBoxMain{\includegraphics[scale=\ScaleMain]{\FilenameMain}}
\savebox\IBoxChild{\includegraphics[scale=\ScaleChild]{\FilenameChild}}
\psset
{
xunit=\dimexpr\wd\IBoxMain/\Column,
yunit=\dimexpr\ht\IBoxMain/\Row,
}
\begin{document}
\begin{pspicture}[showgrid=top](\Column,\Row)
\rput[bl](0,0){\usebox\IBoxMain}
\rput[tr](4.5,4.5){\usebox\IBoxChild}
\end{pspicture}
\end{document}
Edit:
You might need a grid to exactly specify the position of the child figure. Then the following edit should help. Of course you have to turn off the grid at the final stage by changing showgrid=top to showgrid=false.

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{graphicx}
\def\Row{5}
\def\Column{5}
\def\FilenameMain{example-image-A}
\def\FilenameChild{example-image-B}
\def\ScaleMain{1}
\def\ScaleChild{0.25}
\newsavebox\IBoxMain
\newsavebox\IBoxChild
\savebox\IBoxMain{\includegraphics[scale=\ScaleMain]{\FilenameMain}}
\savebox\IBoxChild{\includegraphics[scale=\ScaleChild]{\FilenameChild}}
\psset
{
xunit=\dimexpr\wd\IBoxMain/\Column,
yunit=\dimexpr\ht\IBoxMain/\Row,
}
\addtopsstyle{gridstyle}
{
griddots=0,
gridcolor=red,
subgridcolor=white,
subgriddiv=10,
}
\begin{document}
\begin{pspicture}[showgrid=top](\Column,\Row)
\rput[bl](0,0){\usebox\IBoxMain}
\rput[tr](4.5,4.5){\usebox\IBoxChild}
\end{pspicture}
\end{document}
Edit 2:
Compile the code above with either latex->dvips->ps2pdf sequence or a single call xelatex to obtain a PDF output (namely overlaid.pdf).
Later, from your main TeX input file, you can then import the PDF output (overlaid.pdf) as follows.
% this is the main TeX input file
\documentclass{article}
\usepackage{graphicx}
\begin{document}
\begin{figure}
\centering
\includegraphics[<any options>]{overlaid}
\caption{Overlaid image in action}
\label{fig:overlaid}
\end{figure}
Your text goes here...
\end{document}
Compile this main TeX input file with pdflatex. Note that this workflow makes your project well organized and cleaner.