If you decide to use .png files in your document, it's fine, you could print some test pages to see the result, but remember that along with .jpg, these are raster images.
This means that they have a fixed number of pixels and may (will) lose quality when changing resolution, such as zooming in. So you might want to use bigger resolutions, enough for fitting within the page width.
Alternatively, you might want to include vector images. This is not an easy question, since it all depends on the type of images you're including. But my suggestion is this: some might be recreated in Latex itself using Tikz and/or pgfplots, such as the diagrams. This would be the best solution because you will be able to include the outputs directly in your pdf, which will be lossless (pdf is vectorial as well).
To expand my point about Tikz vs included graphics, I added an example below to show the difference. I created with an
S in it using Tikz.
Then I took a screenshot of it, and included it, and later I also exported the original output (pdf) as png and included it. Then magnified both of them thanks to the
As you can see, while the first one doesn't lose quality, they both look bad when magnified but the exported one looks worse even when not magnified.
Taking a screenshot of it
Exporting pdf as png