The edit box is in the left hand panel under all your details. To turn it on either Quick Links - edit details or UserCP - edit details (both same place really) and tick the box at bottom of page just before save changes.
As far as I understand, it is only a restricting factor on the permission to edit. You could choose to give specific permission for this image to be edited in this thread and have your edit box still crossed.
There those on here who post photo's in a way that they want them to appear and would (rightly) be quite offended and angry if people messed with colour balance, aspect ratio or anything else for that matter.
There are others who don't mind people editing their pictures, to show where they think they could be improved but have missed the option, for allowing editing, to be turned on.