There are two problems here. First, you forgot the unit of your width. Second, You are nesting optional arguments. When reading an optional argument starting with [, TeX looks for the next ] and considers everything in between the optional argument. In your case, the next ] really belongs to the optional argument of \includegraphics, not \item. You can hide the inner argument with a group.
I would recommend using enumitem to change the label for the whole list at once.
\documentclass{article}
\usepackage{mwe}
\usepackage{enumitem}
\begin{document}
\begin{itemize}
\item[{\includegraphics[width=1em]{example-image}}] first sentence
\end{itemize}
\begin{itemize}[label={\includegraphics[width=1em]{example-image}}]
\item first sentence
\end{itemize}
\end{document}