Hi Lizanne
It is preferred you send out IRC's with your request, as it will not only cover the costs of having something returned to you, but is also polite to pay for costs when you're asking for something from someone.
Some celebrities will send you something without sending IRC's, but not everyone.
I know IRC's can be very expensive (they're really expensive here in Australia too!) 1 IRC will cover the cost of a regular sized envelope. You will need to send 2 or 3 for an envelope that will fit a large 8x10 photo.
Are you sending postcards or smaller photos to be signed? 1 IRC should probably be enough for your SAE. Celebrities have been known to add extra postage if you haven't sent the full amount.
Additionally, if you have an opportunity you could purchase US stamps (if you're sending requests to the US) from the USPS website which can be found
here. In the long run, the stamps will be cheaper than sending IRC's.
Good luck!
nowandagain [rockon]