Overfull \hbox when \textwidth exceeds \columnwidth while using \includegraphics in a 2 column template

Question

I have a 2 column template in Overleaf.

I would like the figure to span both the columns (otherwise the text in the figure becomes too small).

I tried the following script:

 \begin{figure}
 \includegraphics[width=\textwidth]{Fig1.png}
 \end{figure}

It renders the following result:

The black line on the right is there because of Overfull \hbox.

Interestingly, the Overfull \hbox happens whenever the \includegraphics width exceeds the \columnwidth.

As an example, using \includegraphics[width=0.5\textwidth]{Fig1.png} renders the following image:

Whereas, \includegraphics[width=0.4\textwidth]{Fig1.png} renders correctly:

I have worked on it for hours but to no avail. Please guide me if/when free.