Go to the iPhoto Library, select the thumbnail of the photo you want to view and right-click on it and select "Show File" (I only have iLife '06 but it may still be the same in iPhoto '08).
When the finder window launches showing the original photo, which is nested in the iPhoto Library Folder, select it and double click it. If you can't see it there then it may have been deleted accidentally or not imported into the iPhoto Library Folder in the first place.
Sometimes when you right-click on a photo in the iPhoto Library window and select 'Show File', while a Finder window launches showing the photo's from the Roll .... often you can't exactly see the photo you're seeking cos the window may have a small scroll bar, so just look around the folder for it. If you still can't see the highlighted original, keep the Finder window open and return to the iPhoto Library, right-click on the pic again to select "Show File" and this time the Finder window will spring to the front and you should see a highlighted photo which is the original file you want. As I wrote earlier, just double-click it and it should show up in your iPhoto Library and the question mark will disappear.
Anytime I've seen that question mark, it's always been because I've gone and deleted the original but not the alias in the iPhoto Library window ... while were on the topic, it's important if you chuck any pics from your iPhoto Library window, you've got to right-click on them to select the original file which is nested within iPhoto's folder on your HD. If you don't do this you end up with broken alias' and confusion.