I agree with looking at the official stuff n' all that jazz everyone says. CX Sometimes it makes a really epic photo.
Personally though, that pose you have there is just fine for the character! Let me put a little input of what I think would help.
-The knife. It's facing straight toward the camera. Try holding it sideways or slightly upward to show it off. Bigger is always better! Own that knife, bro! You're a mad, mad killer and that knife is yo' best friend! XD
-Your other arm. It's behind your back or on your hip, right? Well, to make the pose more dynamic, do something with it. Hold it up a la fencing pose if you want to brandish that knife about, as an idea. Or maybe stroke that barber's blade lovingly! Whatever you feel fits with a character, really. Just try and not have an appendage disappear.
-Last but not least, your expression. Now, I don't know much of Sweeney Todd. But what I've seen of him in trailers and posters and online, he seems to have a small range of emotion and that's tough to work with. For a photo, sometimes that doesn't come across well. There's a fine line between emotionless and dazed, eh? So choose an expression and don't be afraid to be bold and overdo it just a tad! Maybe a maniacal grin or a smug smirk. Again, you know the character far more than I. What do you think fits him well?
There's my two cents! I hope I helped (and I hope I made sense. XP)