Steps to reproduce:
1. Add a grid with several columns, so there is horizontal scroll bar
2. Add an image to the grid ImageList
3. In the CellFormatting assign the ImageKey of a cell to a value
You will notice that the images are not applied initially. You have to scroll the column where you apply the images out of view and bring it back in to see the images.

WORKAROUND

Instead of setting the ImageKey property, set the Image property to the corresponding image from the image list.