I was surprised to find the images folder full of png's when I originally uploaded all images as jpg. Was this a default I failed to notice when making the site a year ago?
The other way round (PNG –> JPG) may happen, if you checked the option "Convert opaque PNGs to JPG" in "Site Properties/Content".
PNGs, found in the "images" folder of your output files may have 2 reasons:
If I recall correctly, there have been some other situations, when Muse converts JPGs to PNGs in the early times of Muse’s existence: Some effects applied to images, which involve transparency (transparency, shadows, rounded corners, …, …), were converted to PNGs, because the JPG file format doesn’t suppot transparency. But later on, Muse started using the corresponding CSS commands to achieve these effects. So, since 2014, PNG conversion wasn’t necessary any more.