NSMutableRequest and Flickr API for upload images

Question

I use Flickr API for my app to manage user's photo. I decided to make authentication using Apple's classes and it works fine for me. Now my app is authenticated and has all necessary tokens and secret keys so it's possible to perform GET requests with authentication for methods like flickr.photos.search, flickr.test.login and so on. But I spent a few days trying to perform upload using http://api.flickr.com/services/upload/ and their instructions here. They say that request should have argument 'photo' and this parameter should not be included in the signature. That is clear, but I have no idea how to implement this parameter the request.

#import "FlickrUploader.h"
#import "HMACSH1.h"
#import "Prefs.h"
#import "NSString+URLEncode.h"

@implementation FlickrUploader

- (void)uploadPhotoAtPath:(NSString*)filePath {

    NSString *upload_api_url = @"http://api.flickr.com/services/upload/";
    NSString *oauth_nonce = [NSString stringWithFormat:@"%d", 10000000 + arc4random()%1000000];
    NSString *oauth_timestamp   = [NSString stringWithFormat:@"%d", (long)[[NSDate date] timeIntervalSince1970]];
    NSString *oauth_consumer_key = CONSUMER_KEY;
    NSString *oauth_signature_method = @"HMAC-SHA1";
    NSString *oauth_version = @"1.0";
    NSString *oauth_token = [[NSUserDefaults standardUserDefaults] objectForKey:OAUTH_ACCESS_TOKEN_KEY];
//creating basestring to make signature without a 'photo' argument according to API   
    NSMutableString *basestring = [[NSMutableString alloc] initWithCapacity:8];

    [basestring appendFormat:@"&oauth_consumer_key=%@",oauth_consumer_key];
    [basestring appendFormat:@"&oauth_nonce=%@",oauth_nonce];    
    [basestring appendFormat:@"&oauth_signature_method=%@",oauth_signature_method];
    [basestring appendFormat:@"&oauth_timestamp=%@",oauth_timestamp];
    [basestring appendFormat:@"&oauth_token=%@", oauth_token];
    [basestring appendFormat:@"&oauth_version=%@",oauth_version];
//this is may class to make HMAC-SHA1 signature (it works for authentication and for GET requests)
    HMACSH1 *hMACSH1 = [[HMACSH1 alloc] init];

    NSMutableString *urlEncodedBaseString = [[NSMutableString alloc] initWithCapacity:3];
    [urlEncodedBaseString appendString:@"POST"];
    [urlEncodedBaseString appendFormat:@"&%@",[upload_api_url urlEncodedString]];
    [urlEncodedBaseString appendFormat:@"&%@",[basestring urlEncodedString]];


    NSString *oauth_token_secret = [[NSUserDefaults standardUserDefaults] objectForKey:OAUTH_TOKEN_SECRET_KEY];

    NSString *hash_key = [CONSUMER_SECRET stringByAppendingFormat:@"&%@",oauth_token_secret];

    NSString *oauth_signature = [hMACSH1 hmacSH1base64ForData:urlEncodedBaseString keyValue:hash_key];

//creating url for request    
    NSMutableString *urlString = [[NSMutableString alloc] initWithCapacity:8];

    [urlString appendFormat:@"%@",upload_api_url];
    [urlString appendFormat:@"?"];
    [urlString appendString:basestring];
    [urlString appendFormat:@"&oauth_signature=%@", oauth_signature];

    NSURL *authURL = [[NSURL alloc] initWithString:urlString]; 

    NSMutableURLRequest* request = [NSMutableURLRequest requestWithURL:authURL
                                                           cachePolicy:NSURLRequestUseProtocolCachePolicy
                                                       timeoutInterval:30.0];
    request.HTTPMethod = @"POST";

    UIImage *img = [UIImage imageNamed:filePath];
    NSData *imgData = UIImageJPEGRepresentation(img, 0.8);

//here I don't know what to do :((( perhaps NSOutputStream



}