Now images can also be hidden by placing them between the spoiler tags:
[spoiler]Here's an image I want to hide:
[/spoiler]
And absolutely nothing has changed about how you use the tags. Simply type the code for your image(s) as you normally would:
[spoiler]Here's an image I want to hide:
[img width=155 height=118]http://www.yoursite.com/pic.jpg[/img][/spoiler]
This new feature also has the advantage of hiding any smileys that you might use in your spoilers. I've always thought it was rather tacky that the smileys showed even though the text was hidden until the mouse cursor was passed over the spoiler box. However, it didn't dawn on me how to hide images until today. But better late than never.