iOS 图片来源有三种方法：

typedef NS_ENUM(NSInteger, UIImagePickerControllerSourceType) {
    UIImagePickerControllerSourceTypePhotoLibrary,    //1.从图库中选择
    UIImagePickerControllerSourceTypeCamera,          //2.直接调用摄像头拍照
    UIImagePickerControllerSourceTypeSavedPhotosAlbum //3.从相册中选择
} __TVOS_PROHIBITED;

UIImagePickerController是系统提供的用来获取图片和视频的接口，用UIImagePickerController类来获取图片视频，大体分为以下几个步骤：

1. 实例化UIImagePickerController
2. 设置UIImagePickerController数据来源类型
3. 设置代理，遵循UIImagePickerControllerDelegate,UINavigationControllerDelegate协议
4. 实现代理方法接收选取图片并做处理

在代理中获取我们选中的图片，UIImagePickerControllerDelegate代理中一共三个方法，其中一个3.0已经废弃了，只剩下两个我们需要用的。

//1.当用户选取完成后调用；
- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info;

//2.当用户取消选取时调用；
- (void)imagePickerControllerDidCancel:(UIImagePickerController *)picker;

在这里，我们主要使用第一个代理方法，在选取图片完成后调用，选取的信息都在info中，info 是一个字典。

// 代理方法,获取图片
// 选取的信息都在info中，info 是一个字典。
/**
// info dictionary keys
UIKIT_EXTERN NSString *const UIImagePickerControllerMediaType __TVOS_PROHIBITED;//指定用户选择的媒体类型
UIKIT_EXTERN NSString *const UIImagePickerControllerOriginalImage __TVOS_PROHIBITED;//原始图片
UIKIT_EXTERN NSString *const UIImagePickerControllerEditedImage __TVOS_PROHIBITED;//修改后的图片
UIKIT_EXTERN NSString *const UIImagePickerControllerCropRect __TVOS_PROHIBITED;//裁剪尺寸
UIKIT_EXTERN NSString *const UIImagePickerControllerMediaURL __TVOS_PROHIBITED;//媒体的URL
UIKIT_EXTERN NSString *const UIImagePickerControllerReferenceURL        NS_AVAILABLE_IOS(4_1) __TVOS_PROHIBITED;//原件的URL
UIKIT_EXTERN NSString *const UIImagePickerControllerMediaMetadata       NS_AVAILABLE_IOS(4_1) __TVOS_PROHIBITED;//如果是拍照的照片，则需要手动保存到本地，系统不会自动保存拍照成功后的照片

下面是具体的代码，只写了核心的部分，当你需要获取系统图片时，执行- (void)startGetPhoto;此方法即可。

//点击更换头像时开始执行此方法
- (void)startGetPhoto
{
    self.actionSheet = [[UIActionSheet alloc] initWithTitle:nil
                                                   delegate:self
                                          cancelButtonTitle:@"取消"
                                     destructiveButtonTitle:nil
                                          otherButtonTitles:@"相册", @"拍照", nil];
    self.actionSheet.tag = ActionSheetTagAddPhoto;
    [self.actionSheet showInView:self.view];
}

#pragma mark -
#pragma mark UIActionSheetDelegate Call Back Implementation

- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex
{
       if (0 == buttonIndex)
    {
        //打开本地相册
        [self localPhoto];
    }
    else if(1 == buttonIndex)
    {
        // 开始拍照
        [self takePhoto];
    }

}

#pragma mark -
#pragma mark UIImagePickerControllerDelegate Call Back Implementation

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
    NSString *type = [info objectForKey:UIImagePickerControllerMediaType];

    //当选择的类型是图片
    if ([type isEqualToString:@"public.image"])
    {
        NSString *key = nil;

        if (picker.allowsEditing)
        {
            key = UIImagePickerControllerEditedImage;
        }
        else
        {
            key = UIImagePickerControllerOriginalImage;
        }
        //获取图片
        UIImage *image = [info objectForKey:key];

        if (picker.sourceType == UIImagePickerControllerSourceTypeCamera) {
            // 固定方向
            image = [image fixOrientation];//这个方法是UIImage+Extras.h中方法
            //压缩图片质量
            image = [self reduceImage:image percent:0.1];
            CGSize imageSize = image.size;
            imageSize.height = 320;
            imageSize.width = 320;
            //压缩图片尺寸
            image = [self imageWithImageSimple:image scaledToSize:imageSize];
        }
        //上传到服务器
        //[self doAddPhoto:image];
        //关闭相册界面
        [picker dismissViewControllerAnimated:YES completion:^{

        }];
    }
}

// 开始拍照
-(void)takePhoto
{
    if ([UIImagePickerController isSourceTypeAvailable:UIImagePickerControllerSourceTypeCamera])
    {
        UIImagePickerController *picker = [[UIImagePickerController alloc] init];
        picker.sourceType = UIImagePickerControllerSourceTypeCamera;
        picker.delegate = self;
        //设置拍照后的图片可被编辑
        picker.allowsEditing = YES;
        picker.sourceType = UIImagePickerControllerSourceTypeCamera;
        //先检查相机可用是否
        BOOL cameraIsAvailable = [self checkCamera];
        if (YES == cameraIsAvailable) {
            [self presentViewController:picker animated:YES completion:nil];
        }else {
            UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"提示" message:@"请在iPhone的“设置-隐私-相机”选项中，允许本应用程序访问你的相机。" delegate:self cancelButtonTitle:@"好，我知道了" otherButtonTitles:nil];
            [alert show];
        }

    }
}

// 打开本地相册
-(void)localPhoto
{    
    //本地相册不需要检查，因为UIImagePickerController会自动检查并提醒
    UIImagePickerController *picker = [[UIImagePickerController alloc] init];
    picker.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
    picker.delegate = self;
    //设置选择后的图片可被编辑
    picker.allowsEditing = YES;
    [self presentViewController:picker animated:YES completion:nil];
}

    //检查相机是否可用
- (BOOL)checkCamera
{
    NSString *mediaType = AVMediaTypeVideo;
    AVAuthorizationStatus authStatus = [AVCaptureDevice authorizationStatusForMediaType:mediaType];
    if(AVAuthorizationStatusRestricted == authStatus ||
       AVAuthorizationStatusDenied == authStatus)
    {
        //相机不可用
        return NO;
    }
    //相机可用
    return YES;
}

//压缩图片质量
-(UIImage *)reduceImage:(UIImage *)image percent:(float)percent
{
    NSData *imageData = UIImageJPEGRepresentation(image, percent);
    UIImage *newImage = [UIImage imageWithData:imageData];
    return newImage;
}
//压缩图片尺寸
- (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize
{
    UIGraphicsBeginImageContext(newSize);
    [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}