iOS: Some files don't open - ios

Question

I am attempting to load image files as an NSString, but all of them come up nil using this code:
NSString *path = [[NSBundle mainBundle] pathForResource:[NSString stringWithUTF8String:name.data()] ofType:nil];
NSString *da = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];
I am able to load many files, but all JPEG and PNG files fail for some reason. I thought it might have something to do with encoding so I switched it to usedEncoding, but it still didn't work.
What am I missing?
EDIT:
I have been making an iOS/Android cross platform OpenGL graphics library in C++. Everything works except texture loading. Any file loading from disk goes through one function that is abstracted between systems. I need the image file in an STL string, so that I can pass it to an image parsing library to get the raw pixel data.
I just think that it's reduculous that the function I have can open any file except images.

Answer 1

If you run your code, passing an NSError instance instead of nil,
NSError *error = nil;
NSString *string = [NSString stringWithContentsOfFile:filePath
encoding:NSUTF8StringEncoding
error:&error];
you will see that stringWithContentsOfFile cannot open the image file, returning nil and the error given is:
Error Domain=NSCocoaErrorDomain Code=261 "The operation couldn’t be completed. (Cocoa error 261.)"...
Cocoa error 261 is NSFileReadInapplicableStringEncodingError which means the encoding of the file is different from the one you are passing (NSUTF8StringEncoding). But I have tried with the other encodings, and none works for PNG files.
You can still achieve what you want by loading the file as a UIImage and then converting the UIImage into a Base64 string.
Since iOS 7, this is easier because you can use the built in method base64EncodedStringWithOptions:
// Load the image and convert it to NSData
UIImage *image = [UIImage imageNamed:#"yourImageName"];
NSData *imageData = UIImagePNGRepresentation(image);
// You can use the equivalent UIImageJPEGRepresentation() for JPEG images
// Convert NSData to a Base64 NSString
NSString *base64ImageString = [imageData base64EncodedStringWithOptions:NSDataBase64Encoding64CharacterLineLength];
Previous to iOS 7, you can do the exact same thing but you will have to implement your own Base64 encoding method (Or import any of the many already available, eg. nicklockwood/Base64).