\documentclass[fleqn]{article}
\usepackage{graphicx}
\usepackage{marginnote}\def\marginparsep{3cm}\reversemarginpar\def\mark{\marginnote{\includegraphics[width=1cm]{test.png}}}
\begin{document}
Hello world
\mark \[x^2+1\]
Hello world
\vspace{2cm}\\
\mark Hello world 2
\end{document}