Latex: Insert vertical space after includegraphics

To get the width of the image you can use this code:

\newlength{\imgwidth}
\settowidth{\imgwidth}{\includegraphics{img}}

You could use this in the document preamble to create a new command to automatically set the width:

\usepackage{graphicx}
\usepackage{calc}

\newlength{\imgwidth}

\newcommand\scalegraphics[1]{%   
    \settowidth{\imgwidth}{\includegraphics{#1}}%
    \setlength{\imgwidth}{\minof{\imgwidth}{\textwidth}}%
    \includegraphics[width=\imgwidth]{#1}%
}

and then, in your document:

\scalegraphics{img}

I hope this helps!

It seems to me that what you really want, is to customize the look of the captions. This is most easily done using the caption package. For instructions how to use this package, see the manual (PDF). You would probably need to create your own custom caption format, as described in chapter 4 in the manual.

Edit: Tested with MikTex:

\documentclass{report}

\usepackage{color}
\usepackage{xcolor}
\usepackage{listings}

\usepackage{caption}
\DeclareCaptionFont{white}{\color{white}}
\DeclareCaptionFormat{listing}{\colorbox{gray}{\parbox{\textwidth}{#1#2#3}}}
\captionsetup[lstlisting]{format=listing,labelfont=white,textfont=white}

% This concludes the preamble

\begin{document}

\begin{lstlisting}[label=some-code,caption=Some Code]
public void here() {
    goes().the().code()
}
\end{lstlisting}

\end{document}

Result: