If you find that the image display ordering seems incorrect when you open a drawing, select an image, or use an editing command, then you can use REGEN to restore the proper order. The image display order is affected by the following conditions:
AutoCAD respects image insertion order first. For example, if you insert images A, B, C in that order, and then reorder them so that they are arranged as B, C, A, then you need to use REGEN to restore the B, C, A order when you open the drawing again. This is an optimization feature that AutoCAD applies to the display order of all objects, not just to images.
When you use an editing command on an image, that image may rise to the top of the display order. Use the AutoCAD REGEN command to restore the proper display order.
For more information, see Changing the Display Order of Images.