gistfile1.m

- (NSURL *) checkURLonImage :(NSString *) artworkStringURL {
    
    // make strings
    
    NSString *artwork600 = [artworkStringURL stringByReplacingOccurrencesOfString:@"100x100-75"
                                                                          withString:@"600x600-75"];
    
    NSString *artwork400 = [artworkStringURL stringByReplacingOccurrencesOfString:@"100x100-75"
                                                                       withString:@"400x400-75"];
    
    // urls
    NSURL *urlFor600 = [NSURL URLWithString:artwork600];
    NSURL *urlFor400 = [NSURL URLWithString:artwork400];
    
    // data
    NSData *data600 = [NSData dataWithContentsOfURL:urlFor600];
    NSData *data400 = [NSData dataWithContentsOfURL:urlFor400];
    
    if (data600 != nil) {
        return urlFor600;
    }
    else if (data400 != nil)
    {
        return urlFor400;
    }
    else
    {
        return nil;
    }
    
}