When the ButtonType property of a CommandField field is set to ButtonType.Image, use the UpdateImageUrl property to specify the image to display for an Update button. This image can be in any file format (.jpg, .gif, .bmp, and so on), as long as the client's browser supports that format.
As an alternative to displaying an image for an Update button, you can display text by first setting the ButtonType property to ButtonType.Button or ButtonType.Link and then setting the UpdateText property.
The following code example demonstrates how to use the UpdateImageUrl property to specify a custom image to display for an Update button in a CommandField field of a GridView control. The ButtonType property must be set to ButtonType.Image for an Update button to display an image.