Very odd...mine are all taken as jpg and I didn't think it was possible to modify the file type of images taken with the camera except on an HTC device.
Why would you want jpg instead of png?
If you INSIST on jpg for some reason, you could use an image converter from play Store, but jpg has some pretty incredible quality issues, especially when compared to png.
I am certain after some digging that images captured through the camera can only be saved as jpg but you are correct, screenshot images are processed as png. There is no way to change this, not even with a 3rd party camera app.
I keep mine set at 8mp for everyday use....Works really well for me. I almost never turn my phone sideways while shooting tho...so It might be different for other ppl!