To use/include a figure within an Overleaf document you first have to upload it via the project menu. You can then specify the size of the image in the output document using the options to the includegraphics command:

\includegraphics[width=0.5\textwidth]{my-uploaded-figure.png}


There's an example of this in our default paper template—try it out by creating a new paper to see how the image 'frog.jpg' is set to half the text width.