-(void)postTweet:(NSString*)message withImage:(UIImage*)img {

dispatch_async(GCDBackgroundThread, ^{
    @autoreleasepool {
        NSString *strMsg =[NSString stringWithFormat:@"%@",message];
        NSData *imgData =UIImagePNGRepresentation(img);
        [UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
        NSError *returnCode = [[FHSTwitterEngine sharedEngine]postTweet:strMsg withImageData:imgData];


        [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;

        NSString *title = nil;
        NSString *message = nil;


        if (returnCode) {
            title = [NSString stringWithFormat:@"Error %d",returnCode.code];
            message = returnCode.localizedDescription;

        } else {
            title = @"Tweet Posted";
            message = message;
        }

        dispatch_sync(GCDMainThread, ^{
            @autoreleasepool {
                UIAlertView *av = [[UIAlertView alloc]initWithTitle:title message:message delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
                [av show];
            }
        });
    }
});

}

- (IBAction)btnPost:(id)sender {

    NSString *str =@"Test5";
    UIImage *image =[UIImage imageNamed:@"contact.png"];
    if (image ==nil) {
        [self postTweet:str];
    }else {

    [self postTweet:str withImage:image];

    }
}

- (NSError *)sendPOSTRequestForURL:(NSURL *)url andParams:(NSDictionary *)params
...
[body appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name="%@"rn",key] dataUsingEncoding:NSUTF8StringEncoding]];
[body appendData:[@"rn" dataUsingEncoding:NSUTF8StringEncoding]];
[body appendData:data];

if ([obj isKindOfClass:[NSData class]]) {
[body appendData:[@"rn" dataUsingEncoding:NSUTF8StringEncoding]];
}

- (NSError *)postTweet:(NSString *)tweetString withImageData:(NSData *)theData inReplyTo:(NSString *)irt {
...
NSMutableDictionary *params = [NSMutableDictionary dictionary];
params[@"media[]"] = theData;

params[@"status"] = tweetString;