Command Picture Hanging strips, cheapest I've seen lately is around £3 a pack. Bought some from Wilko last week to hang a fairly large from and it's still going strong (unlike the No More Nails adhesive strips I tried on a lighter picture).
Thought I'd buy some more to hang the other picture and came across these for 3 for 2.
Can attach and reattach as it's similar to velcro.
Link to medium ones.http://www.ebay.co.uk/itm/Brand-New-Command-Damage-Free-Hangin-2-00-/401208441566