For an example, refer to RasterImageListItem.Image.

The preferred way to add multiple items to a RasterImageList is to use the AddRange method of the RasterImageListItemCollection (accessed through the Items property of the RasterImageList). This enables you to add an array of items to the list in a single operation. However, if you want to add items one at a time using the Add method of the RasterImageListItemCollection class, you can use the BeginUpdate method to prevent the control from repainting the RasterImageList each time an item is added. Once you have completed the task of adding items to the control, call the EndUpdate method to enable the RasterImageList to repaint. This way of adding items can prevent flickered drawing of the RasterImageList when a large number of items are being added to the control.

Reference

RasterImageList Class
RasterImageList Members