Sketching up a little drawable context guy.

FMDrawContext

@interface FMDrawContext : NSObject {
    CGContextRef _cgContext;
}

+ (id)drawContextWithSize:(NSSize)s;

- (CGImageRef)CGImage __attribute__((cf_returns_retained));
- (NSImage*)NSImage;
- (NSData*)dataOfType:(NSString*)uti;
- (void)drawInContextWithBlock:(void (^)())r;
- (CGContextRef)context;

@end


@implementation FMDrawContext

+ (id)drawContextWithSize:(NSSize)s {
    
    
    CGColorSpaceRef cs = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
    
    CGContextRef ctx = CGBitmapContextCreate(nil, s.width, s.height, 8, 0, cs, kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Host);
    
    if (!ctx) {
        return nil;
    }
    
    FMDrawContext *drawContext = [[FMDrawContext new] autorelease];
    
    [drawContext setContext:ctx];

    return drawContext;
}

- (void)dealloc {
    
    if (_cgContext) {
        CGContextRelease(_cgContext);
    }
    
    [super dealloc];
}

- (CGImageRef)CGImage {
    return CGBitmapContextCreateImage(_cgContext);
}

- (NSImage*)NSImage {
    
    CGImageRef cimg = [self CGImage];
    NSImage *nimg = [NSImage imageWithCGImage:cimg];
    CGImageRelease(cimg);
    
    return nimg;
}

- (NSData*)dataOfType:(NSString*)uti {
    
    CGImageRef img = [self CGImage];
    
    NSMutableData *data = [NSMutableData data];
    CGImageDestinationRef imageDestination = CGImageDestinationCreateWithData((CFMutableDataRef)data, (CFStringRef)uti, 1, nil);
    
    if (!imageDestination) {
        CGImageRelease(img);
        return nil;
    }
    
    CGImageDestinationAddImage(imageDestination, img, (CFDictionaryRef)[NSDictionary dictionary]);
    CGImageDestinationFinalize(imageDestination);
    CFRelease(imageDestination);
    CGImageRelease(img);
    
    return data;
}

- (CGContextRef)context {
    return _cgContext;
}

- (void)setContext:(CGContextRef)ctx {
    
    NSAssert(!_cgContext, @"There's already a draw context!  This guy is meant to be one-shot.");
    
    _cgContext = ctx;
}

- (void)drawInContextWithBlock:(void (^)())r {
    
    NSGraphicsContext *currentNSContext = [NSGraphicsContext graphicsContextWithGraphicsPort:_cgContext flipped:NO];
    [NSGraphicsContext saveGraphicsState];
    [NSGraphicsContext setCurrentContext:currentNSContext];
    r();
    [NSGraphicsContext restoreGraphicsState];
}

@end

Example for making a quick PNG:

FMDrawContext *dc = [FMDrawContext drawContextWithSize:[img size]];

[dc drawInContextWithBlock:^{
// do your drawing here.
}];

NSData *pngData = [dc dataOfType:(id)kUTTypePNG];

OH DEAR LORD I'M LEAKING THE COLORSPACE UGH