NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);

NSString *filePath = [[paths objectAtIndex:0] stringByAppendingPathComponent:[NSString stringWithFormat:@"a.jpg"]];   // 保存文件的名称

BOOL result = [UIImagePNGRepresentation(original)writeToFile: filePath atomically:YES];

如果成功result为TRUE，filePath即为访问目录