How to add Image to a SQlite database in UWP c#?

I do not recommend saving a byte[] in the database. Saving the image directly in the database only makes sense if the database is accessible by other users with other users who do not have a local file of the image. Converting the image data into a byte[], or Base64, is slower than using the file system.

Instead, as Sunteen mentions, save the image to LocalFolder and then you can save a simple string of the file path in the database.

You can easily get the file path from the StorageFile object.

As an example, I've updated your code so that it saves the StorageFile returned from CaptureFileAsync in the app's LocalFolder. Then you can just use that path to set the Image in the UI and save that string to the database item being added.

Using your code, here's the approach:

// a private field on the class level
private string _currentImageFilePath;

private async void btcapture_Click(object sender, RoutedEventArgs e)
{
    var cameraUI = new CameraCaptureUI();
    cameraUI.PhotoSettings.Format = CameraCaptureUIPhotoFormat.Jpeg;
    cameraUI.PhotoSettings.CroppedSizeInPixels = new Size(150, 150);
    var photo = await cameraUI.CaptureFileAsync(CameraCaptureUIMode.Photo);

    if (photo == null) return;

    // Create a unique filename
    var uniqueFileName = $"{Guid.NewGuid()}.jpg";

    // Save the photo file in the app's Local folder
    var savedFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(uniqueFileName, CreationCollisionOption.GenerateUniqueName);

    // -- Now you have an easy to store string using the file's Path property -- //

    // 1 - Use this string to save to the database
    _currentImageFilePath = savedFile.Path;

    // 2 - You can also use the same file path to set the <Image/> source
    // you do NOT need to decode the image manually
    Capture.Source = new BitmapImage(new Uri(savedFile.Path));
}

private async Task SaveToDatabase()
{
    var selectedDate = Data1.Date.UtcDateTime;

    var Db_Pet = new DataBaseHelper();

    if (txtnombre.Text != "" & txtchip.Text != "" & txtraza.Text != "" & Selectedgenero.Text != "" & txtcolor.Text != "")
    {
        var petToInsert = new Mascota
        {
            Name = txtnombre.Text,
            Chip = txtchip.Text,
            ImageFilePath = _currentImageFilePath
        };

        Db_Pet.Insert(petToInsert);
    }
    else
    {
        await new MessageDialog("Enter your pet").ShowAsync();
    }
}

Note that there are some scenarios where saving the data directly in the database makes sense: if you're using that database where the app doesnt have the image files saved locally (think SQL Server with many users, or restore scenarios)