public class ASPxImageGallery :
    ASPxDataViewBase,
    ISupportsImageCaching,
    ISupportsFolderBinding,
    IControlDesigner

Public Class ASPxImageGallery
    Inherits ASPxDataViewBase
    Implements ISupportsImageCaching,
               ISupportsFolderBinding,
               IControlDesigner

<dx:ASPxImageGallery ID="ASPxImageGallery" runat="server"
                     ThumbnailHeight="178px" ThumbnailWidth="178px" ClientInstanceName="ClientImageGallery" >
    <Paddings Padding="0" />
    <SettingsFolder ImageSourceFolder="~/Content/Images/photo_gallery/" ImageCacheFolder="~/Thumb/ImageGalleryThumb/" />
    <SettingsTableLayout ColumnCount="3" RowsPerPage="1" />
</dx:ASPxImageGallery>

using DevExpress.Web;
...
protected void Page_Load(object sender, EventArgs e)
{
    ASPxImageGallery imageGallery = new ASPxImageGallery();
    imageGallery.ID = "imageGallery";
    Page.Form.Controls.Add(imageGallery);

    imageGallery.ClientInstanceName = "ClientImageGallery";
    imageGallery.Paddings.Padding = 0;
    imageGallery.SettingsTableLayout.ColumnCount = 3;
    imageGallery.SettingsTableLayout.RowsPerPage = 1;

    imageGallery.SettingsFolder.ImageSourceFolder = "~/Content/Images/photo_gallery/";
    imageGallery.SettingsFolder.ImageCacheFolder = "~/Thumb/ImageGalleryThumb/";
}

Imports DevExpress.Web

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim imageGallery As ASPxImageGallery = New ASPxImageGallery()
    imageGallery.ID = "imageGallery"
    Page.Form.Controls.Add(imageGallery)

    imageGallery.ClientInstanceName = "ClientImageGallery"
    imageGallery.Paddings.Padding = 0
    imageGallery.SettingsTableLayout.ColumnCount = 3
    imageGallery.SettingsTableLayout.RowsPerPage = 1

    imageGallery.SettingsFolder.ImageSourceFolder = "~/Content/Images/photo_gallery/"
    imageGallery.SettingsFolder.ImageCacheFolder = "~/Thumb/ImageGalleryThumb/"
End Sub

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="ASPxProcessedImageGallery.ascx.cs" Inherits="Watermark.ASPxProcessedImageGallery" %>
3
<dx:ASPxImageGallery ID="ASPxImageGallery" runat="server"></dx:ASPxImageGallery>

<uc1:ASPxProcessedImageGallery runat="server" ID="ASPxProcessedImageGallery" 
  ProcessedImageFolder="~/Images/Processed" SourceImageFolder="~/Images/Original" 
  WatermarkImageUrl="~/Images/watermark.png" >
  <ImageGallery ClientInstanceName="imageGallery"
    ThumbnailWidth="120" ThumbnailHeight="160" Layout="Flow">
    <SettingsFolder ImageCacheFolder="~/Thumb" />
    <SettingsFlowLayout ItemsPerPage="5" />
    <ItemTextTemplate>
      <%# System.IO.Path.GetFileNameWithoutExtension(Container.Item.ImageUrl) %>
    </ItemTextTemplate>
  </ImageGallery>
</uc1:ASPxProcessedImageGallery>

public partial class ASPxProcessedImageGallery : System.Web.UI.UserControl {
  public ASPxImageGallery ImageGallery {
    get {
      return ASPxImageGallery;
    }
  }
  public string ProcessedImageFolder { get; set; }
  public string SourceImageFolder { get; set; }
  public string WatermarkImageUrl { get; set; }
  protected void Page_Init(object sender, EventArgs e) {
    foreach (string item in Directory.GetFiles(Server.MapPath(SourceImageFolder))) {
      if (!File.Exists(SourceImageFolder + "\\" + Path.GetFileName(item)))
        ProcessImage(item);
    }
    ImageGallery.SettingsFolder.ImageSourceFolder = ProcessedImageFolder;
  }

  private void ProcessImage(string name) {
    using (Image image = Image.FromFile(name))
    using (Image watermarkImage = Image.FromFile(Server.MapPath(WatermarkImageUrl)))
    using (Graphics imageGraphics = Graphics.FromImage(image))
    using (TextureBrush watermarkBrush = new TextureBrush(watermarkImage)) {
      int x = (image.Width - watermarkImage.Width);
      int y = (image.Height - watermarkImage.Height);
      watermarkBrush.TranslateTransform(x, y);
      imageGraphics.FillRectangle(watermarkBrush, new Rectangle(new Point(x, y), new Size(watermarkImage.Width + 1, watermarkImage.Height)));
      image.Save(Server.MapPath(ProcessedImageFolder + "\\" + Path.GetFileName(name)));
    }
  }
}

Partial Public Class ASPxProcessedImageGallery
    Inherits System.Web.UI.UserControl

    <PersistenceMode(PersistenceMode.InnerProperty), DesignerSerializationVisibility(DesignerSerializationVisibility.Content)> _
    Public ReadOnly Property ImageGallery() As ASPxImageGallery
        Get
            Return ASPxImageGallery
        End Get
    End Property
    <PersistenceMode(PersistenceMode.Attribute), DesignerSerializationVisibility(DesignerSerializationVisibility.Content)> _
    Public Property ProcessedImageFolder() As String
    <PersistenceMode(PersistenceMode.Attribute), DesignerSerializationVisibility(DesignerSerializationVisibility.Content)> _
    Public Property SourceImageFolder() As String
    <PersistenceMode(PersistenceMode.Attribute), DesignerSerializationVisibility(DesignerSerializationVisibility.Content)> _
    Public Property WatermarkImageUrl() As String
    Protected Sub Page_Init(ByVal sender As Object, ByVal e As EventArgs)
        For Each item As String In Directory.GetFiles(Server.MapPath(SourceImageFolder))
            If Not File.Exists(SourceImageFolder & "\" & Path.GetFileName(item)) Then
                ProcessImage(item)
            End If
        Next item
        ImageGallery.SettingsFolder.ImageSourceFolder = ProcessedImageFolder
    End Sub

    Private Sub ProcessImage(ByVal name As String)
        Using image As Image = System.Drawing.Image.FromFile(name)
        Using watermarkImage As Image = System.Drawing.Image.FromFile(Server.MapPath(WatermarkImageUrl))
        Using imageGraphics As Graphics = Graphics.FromImage(image)
        Using watermarkBrush As New TextureBrush(watermarkImage)
            Dim x As Integer = (image.Width - watermarkImage.Width)
            Dim y As Integer = (image.Height - watermarkImage.Height)
            watermarkBrush.TranslateTransform(x, y)
            imageGraphics.FillRectangle(watermarkBrush, New Rectangle(New Point(x, y), New Size(watermarkImage.Width + 1, watermarkImage.Height)))
            image.Save(Server.MapPath(ProcessedImageFolder & "\" & Path.GetFileName(name)))
        End Using
        End Using
        End Using
        End Using
    End Sub
End Class