Setting the PictureBox.WaitOnLoad property to true means the image is loaded synchronously. This causes the user interface to be blocked from other input until the image is loaded. When PictureBox.WaitOnLoad is false (the default) and the erload:System.Windows.Forms.PictureBox.LoadAsync method is used to load the image, the PictureBox.InitialImage image is displayed when the specified image is loaded, and the user can interact with the interface during the load process.