To reproduce:
RadDesktopAlert oNotificacion = new RadDesktopAlert(this.components);
oNotificacion.FixedSize = new System.Drawing.Size(350, 150);
oNotificacion.CaptionText = "TITLE";
oNotificacion.ContentText = "Alert TEXT";
oNotificacion.ContentImage = this.pictureBox1.Image;

oNotificacion.Show();

Workaround:
oNotificacion.Popup.Image = this.pictureBox1.Image;