My rule of thumb: If the image has few colours, PNG is typically better as you can retain maximum quality with little degradation compared to JPG (e.g. a logo,

My rule of thumb: If the image has few colours, PNG is typically better as you can retain maximum quality with little degradation compared to JPG (e.g. a logo, an illustration). If the image is complex with a lot of colours/shades (e.g. a photograph, an illustration with dozens of colours), JPEG is typically better.