Are you talking about Forever Stamps? You need to weigh the return envelope (in ounces) and check it on the USPS international postage calculator, which is here http://ircalc.usps.gov/

If you're using an ordinary envelope weighing up to 2oz you'll need five, if it weighs less than one ounce it'll be three, but if you're sending something that light and flimsy you might want to add a piece of thin card to protect the photo, which will push the weight up. Some of the envelopes I get back from the US are pretty beaten up around the edges, although a thin piece of card has always protected the photo so far.

Hi there looking into sending some letters out. im from the uk and wondered how many us stamps should i put on the return envelope the envelope is just a little bigger then 10x8 and im only sending one photo to get signed so if i get a reply it will be on photo in the return envelope
how many US stamps should i put on there will 3 be okay?

Thanks

autographcollector11