Simply replace the mentioned LaTeX commands with \includegraphics[<options>]{<image>} (graphicx package). You can adjust the scale or height of the images using the scale and height option, receptively. Using either ex or em as unit for the height will scale the image depending on the font size.
For beamer you do it the following way (See also
Change bullet style / formatting in Beamer)
\documentclass{beamer}
\usepackage{graphicx}
\defbeamertemplate{itemize item}{image}{\small\includegraphics[height=1.6ex]{myimage}}
\defbeamertemplate{itemize subitem}{image}{\scriptsize\includegraphics[height=1.6ex]{myimage}}
\defbeamertemplate{itemize subsubitem}{image}{\tiny\includegraphics[height=1.6ex]{myimage}}
\setbeamertemplate{itemize item}[image]
\setbeamertemplate{itemize subitem}[image]
\setbeamertemplate{itemize subsubitem}[image]
\begin{document}
\begin{frame}{Example}
\begin{itemize}
\item A
\item B
\item C
\begin{itemize}
\item A
\item B
\item C
\begin{itemize}
\item A
\item B
\item C
\end{itemize}
\end{itemize}
\end{itemize}
\end{frame}
\end{document}
For the normal itemize you can redefine \labelitemi, \labelitemii, \labelitemiii and \labelitemiv for the four possible nesting levels.
If you want to have different images for different items you should simply define macros which call \item with the optional argument as described by me in How to define a list with custom symbols?.
For images this would look e.g. like this:
\newcommand*\tick{\item[\includegraphics[height=1.6ex]{tickimg}]}
\newcommand*\fail{\item[\includegraphics[height=1.6ex]{failimg}]}