How do I fix underfull hbox when using custom command?

Question

I've written a small command so that I don't have to keep creating minipages. The command is defined as such:

\newcommand{\itemimage}[3][]{%
    \begin{minipage}{\linewidth}
        \item #2
        
        \centering
        \includegraphics[#1]{#3}
    \end{minipage}
}%

Any time I use this, it seems to generate a warning for "Underfull \hbox (badness 10000) in paragraph"

\begin{enumerate}[topsep=0pt]
    %...
    \itemimage[width=300px]{This is just some text.}{img001.png}
    %...
\end{enumerate}

However, if I don't use the command, and instead use what the command is, then no warning is generated.

\begin{enumerate}[topsep=0pt]
    %...
    \begin{minipage}{\linewidth}
        \item This is just some text.
        
        \centering
        \includegraphics[width=300px]{img001.png}
    \end{minipage}
    %...
\end{enumerate}

What am I missing from my \itemimage command to prevent these warnings from occurring?

Minimal example:

\documentclass[letterpaper,12pt]{extreport}
\usepackage{mathtools}
\usepackage{enumitem}

\edef\restoreparindent{\parindent=\the\parindent\relax}
\usepackage{parskip}
\restoreparindent

\newcommand{\itemimage}[3][]{%
    \begin{minipage}{\linewidth}
        \item #2

        \centering
        \includegraphics[#1]{#3}
    \end{minipage}
}%

\begin{document}
\begin{sloppypar}
\begin{enumerate}[topsep=0pt]
    \itemimage[width=300px]{This is just some text.}{img001.png}
    
    \begin{minipage}{\linewidth}
        \item This is just some text.
        
        \centering
        \includegraphics[width=300px]{img001.png}
    \end{minipage}
\end{enumerate}
\end{sloppypar}
\end{document}

Answer 1

Don't forget to omit spaces at ends of lines, you were generating a line with just a word space which can not stretch to fill the line hence the underfull box. (note you can use example-image in examples as these images are distributed for such tests)

\documentclass[letterpaper,12pt]{extreport}
\usepackage{mathtools}
\usepackage{enumitem}

\edef\restoreparindent{\parindent=\the\parindent\relax}
\usepackage{parskip}
\restoreparindent

\newcommand{\itemimage}[3][]{%
    \begin{minipage}{\linewidth}
        \item #2

        \centering
        \includegraphics[#1]{#3}
    \end{minipage}%<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
}%

\begin{document}
\begin{sloppypar}
\begin{enumerate}[topsep=0pt]
    \itemimage[width=300px]{This is just some text.}{example-image.png}
    
    \begin{minipage}{\linewidth}
        \item This is just some text.
        
        \centering
        \includegraphics[width=300px]{example-image.png}
    \end{minipage}
\end{enumerate}
\end{sloppypar}
\end{document}