Scale image to page width?

Use \textwidth for the width of the text block, and \paperwidth if you want to fit it into the paper width. You could also use \linewidth if you want to fit the image within the line width, which may vary depending on the environment you're in (for example, within a list like enumerate).

Note that if you use \includegraphics outside a figure or table environment, you might want to prepend it with \noindent to avoid the image being pushed over to the right by \parindent. Also, centering the image within the page width (when using \paperwidth) is best obtained using

\begin{center}
  \makebox[\textwidth]{\includegraphics[width=\paperwidth]{...}}
\end{center}

or

\noindent\makebox[\textwidth]{\includegraphics[width=\paperwidth]{...}}

In both instances it typesets a box of width \textwidth, while the contents may stretch outside this width (given by width=\paperwidth). Fixing it to \textwidth avoids Overfull \hbox warnings.

This works as expected within the article and report document class, while some horizontal re-adjustment is required in book. For completeness, and without resorting to page numbering issues that may occur at shipout if the image is placed near a page break, the following \centerimg[<options>]{<image>} command works for all standard document classes, including book:

\documentclass{article}
\usepackage{graphicx,changepage}

\newcommand{\adjustimg}{% Horizontal adjustment of image
  \checkoddpage%
  \ifoddpage\hspace*{\dimexpr\evensidemargin-\oddsidemargin}\else\hspace*{-\dimexpr\evensidemargin-\oddsidemargin}\fi%
}
\newcommand{\centerimg}[2][width=\textwidth]{% Center an image
  \makebox[\textwidth]{\adjustimg\includegraphics[#1]{#2}}%
}

\begin{document}
\mbox{} \par
\noindent\centerimg[width=\paperwidth,height=200pt]{tiger}

\newpage

\mbox{} \par
\noindent\centerimg[width=\paperwidth,height=200pt]{tiger}
\end{document}

The horizontal adjustment for book (obtained via \adjustimg) depends on whether the page number is odd or even. The above MWE, with the tiger image, compiles to the output:

This worked for me

\begin{figure}[ht]
\centering
     \includegraphics[width=1.0\textwidth]{Normal_Case_1_req_1_response}
      \caption{Normal Case: 1 Request \& 1 Response.}
       \label{normal_case}
\end{figure}

I used pdfpages package to include a png. Simple and effective. Fills the entire page with the graphic.

\usepackage{pdfpages}
...
\includepdf{image.png}

Happy TeXing!