Bit depth reduction with dithering is the common solution yet the number of applications that provide this capability is surprisingly low… enter Image Crushr!
Image Crushr is FREE! If you'd like to show your support you can check out my games & apps for iPhone/iPad: kode80 Apps on the AppStore
Image Crushr uses the excellent Sparkle framework by Andy Matuschak for automatic updates: Sparkle Framework
The Image Crushr icon uses glyphs from the free icon set "Icon Sweets" by YummyGum: www.yummygum.nl