In this elementary instance, you can use the following:

\documentclass{article}%
% The [demo] option is just for this example
\usepackage[demo]{graphicx}% http://ctan.org/pkg/graphicx
\newcommand{\HRule}{\rule{\linewidth}{1mm}}
\setlength{\parindent}{0pt}% Just for this example
\begin{document}
\centering
\includegraphics[width = 5.0cm]{logo.png}\par
\kern\dimexpr-\baselineskip-\parskip+2mm\relax
\HRule
\end{document}
Here \kern
inserts vertical space. Since the \Hrule
is set on the baseline and in a new paragraph, the vertical distance is reduced by \baselineskip+\parskip
. An additional 2mm
pushes \Hrule
down far enough to leave a 1mm
gap between the image and the line.
So, for example, if you want a 10bp
gap between the image and your \Hrule
, you would use
\kern\dimexpr-\baselineskip-\parskip+1mm+10bp\relax
In general, if you contain your images inside a figure
environment, you can modify the length \intextsep
. For this, read the layouts
documentation.