public class CustomImageEditDialog : ImageEditorDialog
{
    private RadButton okButton;

    public CustomImageEditDialog() : base()
    {
        this.okButton = new RadButton();
        this.okButton.Parent = this;
        RadButton oldOkButton = this.Controls.Find("radButton1", true)[0] as RadButton;

        this.okButton.Text = oldOkButton.Text;
        this.okButton.Location = oldOkButton.Location;
        this.okButton.Size = oldOkButton.Size;
        this.okButton.Click += OkButton_Click;

        oldOkButton.Hide();
    }

    private void OkButton_Click(object sender, EventArgs e)
    {
        using (System.IO.MemoryStream stream = new System.IO.MemoryStream())
        {
            RadImageEditor radImageEditor1 = this.Controls.Find("radImageEditor1", true)[0] as RadImageEditor;

            radImageEditor1.ImageEditorElement.SaveImage(stream, System.Drawing.Imaging.ImageFormat.Png);
            Telerik.WinControls.RichTextEditor.UI.Size newSize = new Telerik.WinControls.RichTextEditor.UI.Size(radImageEditor1.CurrentBitmap.Size.Width, radImageEditor1.CurrentBitmap.Size.Height);

            Type type = typeof(ImageEditorDialog);
            var inlineField = type.GetField("originalInline", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
            ImageInline originalInline = inlineField.GetValue(this) as ImageInline;

            var actionField = type.GetField("replaceCurrentInlineCallback", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
            Action<Inline, Inline> replaceCurrentInlineCallback = actionField.GetValue(this) as Action<Inline, Inline>;

            ImageInline newInline = new ImageInline(stream, newSize, originalInline.Extension);
            replaceCurrentInlineCallback(originalInline, newInline);
        }

        this.DialogResult = DialogResult.OK;
    }

    protected override void OnClientSizeChanged(EventArgs e)
    {
        base.OnClientSizeChanged(e);

        var c = this.Controls.Find("radButton2", true);
        if (c == null || c.Length == 0)
        {
            return;
        }

        RadButton cancelButton = c[0] as RadButton;
        this.okButton.Left = cancelButton.Left - this.okButton.Width - 10;
        this.okButton.Top = cancelButton.Top;
        this.okButton.Height = cancelButton.Height;
    }
}

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        ImageEditorDialog d = new CustomImageEditDialog();
        this.radRichTextEditor1.RichTextBoxElement.ImageEditorDialog = d;
    }
}