\includegraphics does not find picture if passed as variable

Question

I want to import a picture by passing a variable which stores its name instead of the name itself. Here is example code

\documentclass{article}
\usepackage[margin=0.7in]{geometry}
\usepackage[parfill]{parskip}
\usepackage[utf8]{inputenc}
\usepackage{xparse}
\usepackage{tikz}
\begin{document}
\ExplSyntaxOn
\newcommand{\varpicture}{
    \tl_new:N \l_img_name_tl
    \tl_set:Nn \l_img_name_tl {uniquename.png}
    \node (p) at (0,0) {
        \includegraphics[width=100pt]{uniquename.png} %works
        \includegraphics[width=100pt]{\tl_use:N \l_img_name_tl} %doesn't work
    };

}
\ExplSyntaxOff
\begin{tikzpicture}
    \varpicture
\end{tikzpicture}
\end{document}

Only resource that I found regarding this issue is here: How to use variable for \includegraphics and \attachfile in LaTeX3

If I try to rename the file such that it is called: "uniquename" instead of "uniquename.png" I get the same error, which is that the file cannot be found. None of the solutions used in the other thread work for me.

I compile TeX code with "pdflatex".

Answer 1

A macro at the start of the file name would be expanded once in \includegraphics (as stated by @HeikoOberdiek).

A tl variable can be expected to expand to its contents in one expansion step, so you can drop the \tl_use:N (which needs more than one step) at the beginning and directly use \l_img_name_tl. On recent installations the variant with \tl_use:N would work as well (as also mentioned by @UlrikeFischer)

\documentclass{article}
\usepackage[margin=0.7in]{geometry}
\usepackage[parfill]{parskip}
\usepackage[utf8]{inputenc}
\usepackage{xparse}
\usepackage{tikz}
\begin{document}
\ExplSyntaxOn
\tl_new:N \l_img_name_tl
\newcommand{\varpicture}{
    \tl_set:Nn \l_img_name_tl {example-image-duck.pdf}
    \node (p) at (0,0) {
        \includegraphics[width=100pt]{example-image-duck.pdf} %works
        \includegraphics[width=100pt]{\l_img_name_tl}
    };

}
\ExplSyntaxOff
\begin{tikzpicture}
    \varpicture
\end{tikzpicture}
\end{document}