Just thought I'd add, I love the background on your profile, it looks so awesome, especially with the images. Goes together really well. Also I just woke up when I commented earlier so if my explanation was weird just let me know, lol.
You were wondering about the hidden images I have so basically what I did was set an image in the background and then on top of that image, at about the same size, I have another section. The part that you see appear all of a sudden is column 3, the section on top. In the CSS you just use hover to change the opacity after hovering over it so that it goes from see-through to solid. I have a transition in there to make the fade in/out gradual. Just make sure that for the section that's going to fade in when you hover has a higher z-index than whatever is under it. The code for making the opacity change so that it fades in when you hover is below. You can mess with the opacity after hovering so that it's more or less see-through. I have mine go to 90% opacity so that you can still vaguely make out the library image in the background. This isn't the full code I use for that section but should show you the answer to your question I hope.
transition: opacity 2s linear 0s;
-moz-transition: opacity 2s linear 0s;
-webkit-transition: opacity 2s linear 0s;
-o-transition: opacity 2s linear 0s;