It may be a difference in codes or habits between the U.K. and here in the U.S., but the feed line to any switch or outlet must be attached to the same wall stud that the junction box for the switch or outlet is attached (usually by nails, occasionally by screws). The wire must be stapled to that stud no more that 12 inches from the junction box (to prevent anyone subsequently pulling the wire out causing a short). If the wire feeds a switch, the switch must be 4' above the floor, while if it feeds an outlet they can not be more than 18" above the floor.
Granted, after the wire is attached to the stud it then can be routed horizontally to the next fed device, but those horizontal wires (routed through drilled holes in the intervening studs) would be on a fairly level line between devices.
Long way of saying, the use of a wire detector is great, but the chaeper ones don't work all the time. If your picture is at, say, eye level and there are no outlets or switches to the left or right near that level, one is safe in attaching a picture. Fact is, if the wall is constructed of drywall (gypsum board?) the hanging device shouldn't go through the wall anyway...