From what im aware, PNG generally is good quality in small sizes but looses out when text isnt involved.
JPEG is a worse choice for storing images that require further editing as it suffers from generation loss, whereas lossless formats do not. This makes PNG useful for saving temporary photographs that require successive editing. When the photograph is ready to be distributed, it can then be saved as a JPEG, and this limits the information loss to just one generation. That said, PNG does not provide a standard means of embedding Exif image data from sources such as digital cameras, which makes it problematic for use amongst photographers, especially professionals. TIFF does support it as a lossless format.
So Since PNG was designed to replace GIF, and JPEG gives better results when used on unedited\once edited pics, JPEG is probably the way to go.
http://en.wikipedia.org/wiki/Portable_Network_Graphics#Comparison_with_JPEG