How to position side by side images covering the whole page with no overlap?

Since you are already loading tikz.

\documentclass[12pt]{article}
\usepackage{tikz}
\usepackage[paperwidth=8.5in,paperheight=11in,landscape, margin=0mm, 
left=0cm, right=0cm, top=0cm]{geometry}
\usepackage{graphicx}
\begin{document}
\begin{tikzpicture}[overlay,remember picture]
\foreach \anchor in {north west,south west,north east, south east}
{\node[inner sep=0pt,outer sep=0pt,anchor=\anchor] 
at (current page.center)
{\includegraphics[page=1,width=5.5in,height=4.25in,keepaspectratio]{koala}}; }
\end{tikzpicture}
\end{document}

But you can do without:

\documentclass[12pt]{article}
\usepackage[paperwidth=8.5in,paperheight=11in,landscape, margin=0mm, 
left=0cm, right=0cm, top=0cm]{geometry}
\usepackage{graphicx}
\begin{document}\renewcommand{\arraystretch}{0} 
\noindent\begin{tabular}{@{}c@{}c@{}}
\includegraphics[page=1,width=5.5in,height=4.25in,keepaspectratio]{koala}
&\includegraphics[page=1,width=5.5in,height=4.25in,keepaspectratio]{koala}\\
\includegraphics[page=1,width=5.5in,height=4.25in,keepaspectratio]{koala}
&\includegraphics[page=1,width=5.5in,height=4.25in,keepaspectratio]{koala}
\end{tabular}
\end{document}