When I build my app with Xcode 4 it always show this error message:
CopyPNGFile /Users/michaelchen/Library/Developer/Xcode/DerivedData/iMusicBox-boeawugacmha
Eva Madrazo posted the right answer (at least for problems with interlaced PNGs), but for those having this problem with a bunch of images as I did, you can use mogrify recursively to de-Interlace every file at once.
find ./ -name "*.png" -exec mogrify -interlace none {} \;
Changing the interlace method doesn't appear to affect the image quality, but affects the image size, none will give you a smaller image.
Mogrify comes with ImageMagick and you can install it with homebrew.