Yes!With .png you should save renders not banners otherwise they'll be huge and most foruns don't accept the size! Like when you want to save a banner with some transparent space you can save it as .png so the quality don't drop!
With a good .jpg file (I tried to save in 100% .jpg but the size would get 52kb so you can just save it with 95% of quality) it gets good too! ^^
