You'll need to upload your images onto an image hosting website like Photobucket or Imgur or the like. Once you've done that, you'll need to put insert the link to the picture in IMG tags, and then put those in spoiler tags. Like so:
[SPOILER*][IMG*]Image URL[/IMG][/SPOILER]
(But remember to take out the asterisks in order for it to have effect.)
I feel really dumb, but how do I put the "spoiler" tabs on my page? I want to use those for my pictures that way it isn't as messy. And can I upload them straight from my computer? :P