It's a pretty telling evolution. The artwork went from an artsy, rootsy design to the final version that has the couple holding hands on the cover, walking on a guitar neck.
The problem with this holding hands thing is that they never get together in the movie. Also, according to an interview with Glen Hansard, the final version has his head pasted onto someone else's body, and they have made Marketa Irglova taller! Totally Hollywood crapola. The whole interview is great by the way - from right after the Oscar win.
Here is the part addressing the artwork:
Pitchfork: Speaking of changing the cover, am I imagining things, or did they change the Once poster when it got released on DVD? They changed what you're wearing.
GH: Oh, man. They fuckin' killed it. You're right. They have us holding hands, which we never do in the film! Those legs aren't mine. Those legs are like three times longer than my legs. It's a completely new body. They literally just used my face. I'm wearing a hat in the original picture, so they Photoshopped my head. If you look at my head, my head looks totally weird, because whoever did the Photoshop job was shit. My head looks really weird, they took my hat off, and they gave me an entirely new body. It's completely bizarre. And they made Mar much taller than she really is. You can look at the original cover and then what they did to it and spot all the crappy differences. It's awful. It's a real shame. But at a certain level you've got to let this shit go. I designed the original poster and the cover of the DVD myself. Myself and John like to do things ourselves, and I do a bit of design for the Frames. I designed all the Frames album covers. So I put together the DVD cover and the poster originally. And then they took it and fucking bastardized it. Instead of walking down a street, they stuck us walking down a big guitar.
Pitchfork: Again, it's a different world. The music industry is sketchy enough as it is, but the movie industry is 100 times that.
GH: Yeah, it's just blatant. They don't give a fuck. They want you to look at the DVD cover and get everything from that. It's the opposite of what someone like Criterion would do. They create wonderful art. With a Criterion DVD, you just want to buy it for the box. They do it right. With the bigger-time DVDs...they could have done such a nicer job. But I'm just complaining from a design point of view.