压缩图片

 CGSizesize =CGSizeMake(478,297);

    UIGraphicsBeginImageContext(size);

    [_image drawInRect:CGRectMake(0, 0, 478, 297)];

    UIImage *imagetwo = UIGraphicsGetImageFromCurrentImageContext();

     UIGraphicsEndImageContext();

    [urlMessagesetThumbImage:imagetwo];