PNG's kick ass, that being said, sometimes JPGs are smaller when compressed. Below is an except from Addison Wesley's iphone cook book.

"Xcode automatically optimizes your PNG images using the pngcrush utility shipped with the SDK. (You’ll find the program in the iPhoneOS platform folders in /Developer. Run it  from the command line with the –iphoneswitch to convert standard PNG files to iPhone- formatted ones.) For this reason,use PNG images in your iPhone apps where possible as your preferred image format."

Now go batch resize your image catalogs into PNG format, now.