Photo Library will have ALL your photos in it and Camera Roll will have ALL your pix taken with your iDevice.
Any created albums essentially display a duplicate of the photo so you can organize the photos. But the original still resides in the Library or Camera Roll. You can delete from any location, but the permanent delete is only from Camera Roll or Library.
It may be that once a photo is in an album, you have to delete it from there first and then delete it from the Camera Roll.
It's helpful if you have new vacation pix to show or family photos that you want to be able to get to in a hurry. I don't think the photo actually duplicates; it's more like a stub/shortcut all in one place (album). That's why you can't delete the item in Camera Roll and expect to keep it in an album.
I found this out the hard way a few weeks back. I took some shots of something that will never happen again. I then moved the pictures into an album. I then went to delete the photos from the camera roll and didn't pay attention to the popup thinking it said something about asking if I wanted to delete the photos from the camera roll only and clicked yes. The pictures were gone quicker than a blink.
Those of you saying "it can't be done" just aren't looking hard enough. There is always a solution to everything. Don't be lazy and give up if it doesn't pop up on the first result page of google for your 'convenience'.... Some things take a little bit of effort.
Which probably means the way you can delete the picture from the camera roll and not an album is going to be too long and difficult of a process for you... So don't even bother clicking the link if you don't want to take the chips off your lap, get up, and take 5 minutes to do this.
Don't be so condescending. You just got here. Prior to your post, the last post in this thread is nearly a year old.
There are MR forum threads detailing the process for copying pix to computer, arranging them in created albums of choice, copying back to iPhone/iPad/iPod and then removing them from Camera Roll. People are finding out that Albums are just a viewing convenience & not duplications or taking up extra storage space on their device.
Why don't you read some of the more recent MR threads that actually post instructions in the thread, instead of scolding readers and sending them off to a different site?