NSString *urlString = @"图片地址";
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:urlString]];
UIImage *downLoadImage = [UIImage imageWithData:data]; // 取得图片
// 本地沙盒目录
NSString *path = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
// 得到本地沙盒中名为"downLoadImage"的路径,"downLoadImage"是保存的图片名
NSString *imageFilePath = [path stringByAppendingPathComponent:@"downLoadImage"];
// 将取得的图片写入本地的沙盒中,其中0.5表示压缩比例,1表示不压缩,数值越小压缩比例越大
BOOL success = [UIImageJPEGRepresentation(downLoadImage, 0.5) writeToFile:imageFilePath atomically:YES];
if (success){
NSLog(@"写入本地成功");
}