Good concern regarding the image. The fact that the man is a Christian doesn't mean that appropriating his image without his permission (a civil wrong against privacy) is okay. I wouldn't just assume. I also wouldn't just assume that the artist doesn't have permission :)
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I think that the text is cool, but I don't know about the photo. Maybe if you did an illustration of John Piper instead of a photo. :) The quote is awesome though!
Are you allow to print a picture of John Piper on a shirt without his permission?