public static event AsyncDownloadPolicy.WeakEventHandler<AsyncDownloadPolicy.DownloadingEventArgs> Downloading

Public Shared Event Downloading As AsyncDownloadPolicy.WeakEventHandler(Of AsyncDownloadPolicy.DownloadingEventArgs)

static void Main() {
    // ...
    DevExpress.Data.AsyncDownloadPolicy.Downloading += AsyncDownloadPolicy_Downloading;
    Application.Run(new Form1());
}
private static void AsyncDownloadPolicy_Downloading(object sender, DevExpress.Data.AsyncDownloadPolicy.DownloadingEventArgs e) {
    e.Cancel = !e.Uri.AbsoluteUri.StartsWith(@"https://www.devexpress.com");
}

Shared Sub Main()
    ' ...
    AddHandler DevExpress.Data.AsyncDownloadPolicy.Downloading, AddressOf AsyncDownloadPolicy_Downloading
    Application.Run(New Form1())
End Sub
Private Shared Sub AsyncDownloadPolicy_Downloading(ByVal sender As Object, ByVal e As DevExpress.Data.AsyncDownloadPolicy.DownloadingEventArgs)
    e.Cancel = Not e.Uri.AbsoluteUri.StartsWith("https://www.devexpress.com")
End Sub

public Form1() {
    InitializeComponent();
    AsyncDownloadPolicy.Downloading += AsyncDownloadPolicy_Downloading;
}
Dictionary<int, string> whitelist = new Dictionary<int, string>() {
    { 0, "www.devexpress.com" },
    { 1, "community.devexpress.com" },
    { 2, "docs.devexpress.com" },
    { 3, "supportcenter.devexpress.com" }
};
private void AsyncDownloadPolicy_Downloading(object sender, AsyncDownloadPolicy.DownloadingEventArgs e) {
    string result = e.ValueType.Name;
    // Checks whether a picture editor initiated the download.
    if(result == "ImageOrSvgImageResult")
        // Cancels downloading from domains that are not in the white list.
        e.Cancel = !whitelist.Values.Contains(e.Uri.Authority);
}
private void loadImageButton_Click(object sender, EventArgs e) {
    pictureEdit1.LoadAsync(/* YOUR_URL */);
}

Public Sub New()
    InitializeComponent()
    AddHandler AsyncDownloadPolicy.Downloading, AddressOf AsyncDownloadPolicy_Downloading
End Sub
Private whitelist As New Dictionary(Of Integer, String)() From {
    { 0, "www.devexpress.com" },
    { 1, "community.devexpress.com" },
    { 2, "docs.devexpress.com" },
    { 3, "supportcenter.devexpress.com" }
}
Private Sub AsyncDownloadPolicy_Downloading(ByVal sender As Object, ByVal e As AsyncDownloadPolicy.DownloadingEventArgs)
    Dim result As String = e.ValueType.Name
    ' Checks whether a picture editor initiated the download.
    If result = "ImageOrSvgImageResult" Then
        ' Cancels downloading from domains that are not in the white list.
        e.Cancel = Not whitelist.Values.Contains(e.Uri.Authority)
    End If
End Sub
Private Sub loadImageButton_Click(ByVal sender As Object, ByVal e As EventArgs)
    pictureEdit1.LoadAsync("YOUR_URL")
End Sub