By default, the iPhone takes Camera photos using JPG image format. If you take a screenshot with your iPhone (hold sleep/wake + home button) the picture format will be PNG. The most probably reason Apple choose to do this – PNG files are better compressed for text screenshots while JPG format is better compressed for photographs that have rich colors.
It’s advised to keep the default image format of pictures produced by iPhone. If you want to convert the images after taking them on your iPhone, install JPG <-> PNG om your iPhone.
All else, use image capture to export the images to your Mac and convert the images with an application such as Mac X Graphic Conversion Software.