I have a 32GB iPhone 4 version 4.0.1. I have many photos on my phone, but I have deleted an important photo from my iPhone. How can I get this photo back from my iPhone?
Unless you have synced your iPhone and thus have your photo somewhere in your computer, or uploaded it to some service you can't recover it easily.
Most likely you don't have any copies of it and you'll have to find some professional to recover it from the iPhone memory. Maybe, and I say really just maybe, apple care support would be able to do it.
Here's one thing you could try, though: manage to get access to iPhone file system somehow (jailbreak, iPhone Explorer, anything you want) and get some file recovery software. Keep in mind recovering deleted files are complicated and expensive business. You'll only be able to do it this way if you get lucky.