Overlaying a full page with an image

A TikZ solution as you alluded to:

\documentclass{report}
\usepackage{tikz}  
\usepackage{mwe} % this package provides dummy images for illustration purposes

\begin{document}
\lipsum[1-2]

% 1st image
\clearpage
\tikz[remember picture,overlay]{\node[]at(current page.center){\includegraphics{example-image-letter-numbered}};} % replace example-image* with your own image file

% 2nd image    
\clearpage
\tikz[remember picture,overlay]{\node[]at(current page.center){\includegraphics{example-image-letter-numbered}};} % replace example-image* with your own image file

\end{document}

---

Edit in response to comment:

The section heading doesn't show because it covered by the image. On way to fix this is to issue the \section command after placing the first image, and this requires temporarily suspending the \sectionbreak command:

\documentclass{report}
\usepackage{tikz}  
\usepackage{mwe} % this package provides dummy images for illustration purposes

\usepackage{titlesec} %allows for more robust sectioning
\newcommand{\sectionbreak}{\clearpage} %starts each section on new page

\begin{document}
\lipsum[1-2]

% 1st image
\clearpage
\tikz[remember picture,overlay]{\node[]at(current page.center){\includegraphics{example-image-letter-numbered}};} % replace example-image* with your own image file

{
\renewcommand\sectionbreak{} % temporarily suspends section break
\section{Flowcharts} % add section title after image so that it won't be covered 
}

% 2nd image    
\clearpage
\tikz[remember picture,overlay]{\node[]at(current page.center){\includegraphics{example-image-letter-numbered}};} % replace example-image* with your own image file

\end{document}

Perhaps this is overkill for your use case, but I wrote a package called pdfoverlay to easily overlay text on a multi-page PDF file. But it works just as well for single page PDFs and even single page PNGs.

Try something like this:

\documentclass{report}

\usepackage{pdfoverlay}

\begin{document}

This is page 1.

\clearpage

\pdfoverlaySetPDF{example-image-letter.pdf}

\chapter{This is a chapter heading}

\clearpage

\pdfoverlaySetPDF{example-image-a4.pdf}
\null % You need some object on the page for LaTeX to output the page.

\clearpage

\end{document}