When you hide an image, the image does not display onscreen, nor does it plot. Only the image boundary displays onscreen. However, the image is still loaded in memory, and the image file is still locked on the disk. It cannot be deleted or modified. Hiding images is a convenient way to speed regeneration time. You can redisplay the image when you are ready to plot.
The images are also affected by other layer attributes such as whether a layer is locked or frozen. If you are working with several images and you do not want to modify certain images, then move those images to a separate layer and lock the layer.