ok i figure out that i have to save the 256 color png to be able to make part of picture transparent.
As I see cocos2d is currently supporting jpg/jpeg, pvr, png format. I think there is no alpha channel in jpg/jpeg and am confused by a comment in code: // Issue #886: TEMPORARY FIX FOR TRANSPARENT JPEGS IN IOS4. ?
And to color with rgba8, should we then use pvr format?
Help will be appreciated.