I hardly ever use png, jpeg compression is awsome for large full colour work. (and invisable to the human eye if set correctly) Did you know you can compress just certain areas of a jpeg, like the background. for print you need at least 300 pixels per inch(300dpi) printed, stick to that and you wont go wrong. even better is 600dpi for crisper images. Tiff is the best for sending to printers(loss-less compression). Jpeg is also good in CMYK mode (but set compression to max or max-1)