Designed book cover

Last night I was sitting in front of my computer and playing around with GIMP (the open source equivalent of Photoshop) and I felt inspired so I created a mock up of what I envision my book cover to be for my current novel-in-progress, That Warrior Spirit.

I really like the design I came up with and I am pleased with myself. I'd love to post it here and show it off but I am faced with an ethical, maybe legal, dilemma. One of the images I used in my composition is a photo I took of one of the men in MDI, with my cell phone. Do I need his permission to use a photo I took of him myself on my book cover? Do I need to get him to sign a model release?

I've looked up model release on wikipedia. It says that Publishing an identifiable photo of a person without a model release signed by that person can result in civil liability for whoever publishes the photograph. One of the key words here seems to be identifiable. If I manipulate the image so that the subject is not recognizable, would I still be at risk of civil liability if I did not have a signed model release?