PNG images ok on simulator but don't display on device
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My iphone app pulls in about 300 images from a bundle, and then on one of my view controllers it displays one at random.  This works fine on the simulator (xcode 4.3.3 / iphone 5.0 or 5.1 simulator) but not when I push it to my device (iphone 3gs).  The app runs and there are no errors on the log, but that part of the view where the image should be is just blank, like the color of the background.

Things I've tried:

 1. I've checked and double-checked the case of the filenames vs
    references in code.   
 2. I checked the Build Phase called Copy Bundle
    Resources, and it's definitely set to copy the bundle to the device.
 3. I looked at the log produced by the build, and although that was the first time I'd examined it in detail, it had no errors and seemed to be copying the bundle across.  Here's the snippet from the log: 
        
        CpResource FlagQuiz/flagimages.bundle /Users/ian/Library/Developer/Xcode/DerivedData/FlagQuiz-drrjrlxfifrmbnaissqfqxuixchb/Build/Products/Debug-iphoneos/FlagQuiz.app/flagimages.bundle
        cd /Users/ian/Documents/xcodeapps/FlagQuiz
        setenv PATH "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin:/Applications/Xcode.app/Contents/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
        builtin-copy -exclude .DS_Store -exclude CVS -exclude .svn -exclude .git -exclude .hg -resolve-src-symlinks -strip-tool /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip /Users/ian/Documents/xcodeapps/FlagQuiz/FlagQuiz/flagimages.bundle /Users/ian/Library/Developer/Xcode/DerivedData/FlagQuiz-drrjrlxfifrmbnaissqfqxuixchb/Build/Products/Debug-iphoneos/FlagQuiz.app

Any ideas?