cyy:自我理解，不保证正确，但亲自试验确实可行：若用 stringByAppendingString 则需要手动在名称前加 “/”符号，而stringByAppendingPathComponent则不需要，它会自动添加
stringByAppendingPathComponent和stringByAppendingString的区别：
NSString *imagePath = [skinPath stringByAppendingString:[NSString stringWithFormat:@"/%@",imageName]];//stringByAppendingString是在skinPath加后缀的意思
NSString *imagePath = [skinPath stringByAppendingPathComponent:imageName];//stringByAppendingPathComponent是在skinPath后面加上“／”号连接imageName让它成为完整的路径
NSLog(@"imagePath:%@",imagePath);
打印：
2013-11-27 14:44:30.052 Willing[2757:70b] imagePath:/Users/will/Library/Application Support/iPhone Simulator/7.0.3/Applications/DA5B603D-4D07-4425-B7CC-5D49232189BE/Willing.app/tabbar_home.png