An Xcode precompilation script to turn your images into auto-completeable, type-checkable symbols.

image.py

import os.path as path
import string
import argparse
import glob
import re

def basename(filename):
    base = filename
    if filename.find('@2x') > 0:
        base = filename[:filename.find('@2x')]
    elif filename.find('~') > 0:
        base = filename[:filename.find('~')]
    elif filename.find('.') > 0:
        base = filename[:filename.find('.')]
    return base

def file_type(filename):
    isRetina = False
    isIpad = False
    if filename.find('@2x') > 0:
        isRetina = True
    if filename.find('~ipad') > 0:
        isIpad = True
    
    if isRetina:
        if isIpad:
            return 'ipad2x'
        else:
            return 'iphone2x'
    else:
        if isIpad:
            return 'ipad'
    return 'iphone'
    
def mungedName(basename):
    parts = re.split('[-_]', basename)
    capitalized = [word.capitalize() for word in parts]
    return string.join(capitalized, '')
    
class ImageGroup:
    iphone = None
    iphone2x = None
    ipad = None
    ipad2x = None
    extras = []
    
    def __init__(self, file_name):
        setattr(self, file_type(file_name), file_name)
                
    def add_file(self, file_name):
        type = file_type(file_name)
        
        if getattr(self, type) is not None:
            self.extras.append(file_name)
        else:
            setattr(self, type, file_name)
    
    def warnings(self, iPhone=True, iPad=True, retina=True, duplicates=True):
        definition = ''
        if iPhone and self.iphone is None:
            definition += '#warning image formatted for iPhone %s not found\n' % filename
        if iPhone and retina and self.iphone2x is None:
            definition += '#warning image formatted for retina iPhone %s not found\n' % filename
        if iPad and self.ipad is None:
            definition += '#warning image formatted for iPad %s not found\n' % filename
        if iPad and retina and self.ipad2x is None:
            definition += '#warning image formatted for retina iPad %s not found\n' % filename
        if duplicates:
            for file in self.extras:
                definition += '#warning duplicate image %s found in project. Verify proper capitalization.\n' % file
        return definition
        
    def output(self, filename, prefix):
        return args.format % {'prefix':prefix, 'identifier':mungedName(filename), 'filename':filename}
        
        
DEFAULT_FORMAT = '#define %(prefix)s%(identifier)s (UIImage*)^{\
 UIImage *image = [UIImage imageNamed:@"%(filename)s"];\
 ZAssert(image, @"Image %(filename)s not found");\
 return image;\
 }()\n\n'

### cmd folder outputFile
parser = argparse.ArgumentParser(description='Create a header file with contants for each image file in the given folder.')
parser.add_argument('-s', '--source', type=str, default='./', help='A folder which contains images.')
parser.add_argument('-d', '--destination', type=str, default='./images.h', help='The filename to write to.')
parser.add_argument('--prefix', type=str, default='img', help='The prefix added at the begining of each image\'s filename.')
parser.add_argument('--format', type=str, default=DEFAULT_FORMAT, help='The format string specifying how the file should be written')
parser.add_argument('--warn-retina', dest='retina', type=bool, default=True, help='Warn for missing retina images.')
parser.add_argument('--warn-ipad', dest='ipad', type=bool, default=False, help='Warn for missing iPad (~ipad) images.')
parser.add_argument('--warn-iphone', dest='iphone', type=bool, default=False, help='Warn for missing iPhone (~iphone) images')
parser.add_argument('--warn-duplicates', dest='duplicates', type=bool, default=True, help='Warn for duplicate images.')

args = parser.parse_args()
source = path.join(path.expanduser(args.source), '*.png')
iterator = glob.iglob(source)
all_files = {}

for fullpath in iterator:
    (leading, filename) = path.split(fullpath)
    key = basename(filename)

    if key in all_files:
        current_file = all_files[key]
        current_file.add_file(filename)
    else:
        current_file = ImageGroup(filename)
        all_files[key] = current_file

### Florian Bruger ensures the file isn't updated needlessly.
original_output = ''
file_path = path.join(path.expanduser('.'), args.destination)
if path.exists(file_path):
    with open(file_path, 'r') as file:
        original_output += file.read()
        file.close()

output = '// Created using the image.py script written by Patrick Hughes. He\'s a pretty cool guy.\n'
output +='//\n// DO NOT EDIT THIS FILE. \n//\n'
output +='// This file is automatically generated. Any changes may be overwritten the next time images.py is invoked.\n\n'

for key in all_files:
    image_group = all_files[key]
    warnings = image_group.warnings(iPhone=args.iphone, iPad=args.ipad, retina=args.retina, duplicates=args.duplicates)
    output += warnings
    output += image_group.output(key, args.prefix)

if original_output == output:
    pass
else:
    with open(file_path, 'w') as file:
        file.write(output)
        file.close()

Super useful :)
Only problem: When including the file inside the prefix.pch, Xcode thinks the file did change even when there are no new images.
I modified the script to only write to the file if the output is different than the file content.. I am not a python expert, maybe there is a better way but for now it works well
https://gist.github.com/4672086

If you set the script up as a dependent project, as outlined in the blog post, it creates the images.h file before the pre-compilation step, which avoids the warnings. Nonetheless I've incorporated your test to the script for efficiency's sake.

Glad I found this, great idea!

My images aren't all in a single folder, so I hacked on it a bit and made it recursive. It assumes you use groups, not folder references, as it uses the basename of the image, not the full path from the source directory passed to the script. Although, now that I think of it, an automated solution like this might make folder references feasible! Should be simple to use the full path as the key instead of the basename, if that's what you desire.

Python isn't my weapon of choice, so let me know if there are any egregious trespasses.

https://gist.github.com/rhgills/5847456