Inserting PNG file as is

This question cannot have general answer for ALL monitors.

The size of the PNG image on the screen depends on the screen resolution (e.g. 110.27 PPI (pixels per inch) for my MacBook), and on Retina (220 PPI) it will be twice smaller.

And the size of image inside PDF, shown at real 100%, will be monitor-independent.

Thus solution for my monitor:

\includegraphics[scale=0.65]{screenshot.png}

will be different from yours:

\includegraphics[scale=0.75]{screenshot.png}