As Rachyn already noted, those of you outside the UK will need to enclose International Reply Coupons. The number you'll need depends on the size of the return envelope.
The US and Canada have yet to adopt ISO paper sizes. The equivalent of US Letter (8.5" x 11") is A4 (210 x 297 mm, or 8.3" x 11.7"), which requires and extra fold to fit a US Letter envelope. A5 is 210 x 148 mm, or 8.3" x 5.8". To keep that photo of Rachel from being delivered with a crease across her nose, you'll need to provide an 8.5" x 11" envelope for the reply. This will have quite a bit of slack space, so maybe address your envelope so it may be folded in half around the card and glued down.
