\documentclass{article}
\usepackage{graphicx}
\DeclareRobustCommand\fool{lower: \includegraphics[height=1ex]{example-image-a}}
\DeclareRobustCommand\foou{upper: \includegraphics[height=1em]{example-image-b}}
\makeatletter
\g@addto@macro\@uclclist{\fool\foou}
\makeatother
\begin{document}
\fool
\MakeUppercase{\fool}
\end{document}