You may get a 303 error when trying to use custom icons in your AIR application (even when your icon files are in the right place).
Assume you are using Flex Builder. First move your icon files out of your source directory (e.g. move to Desktop). Then drag your icon files into Flex Builder and drop to your project directory. Do not simply copy icon files to your project directory using your operating system’s copy utility (Mac Finder or Windows explorer).
Also check out this thread in Adobe AIR forum