They may or may not pay for their own postage and provide their own picture. There are some cases when they do, but you can never be too sure. You just increase your chances of success when you pay for postage yourself and send them a picture. I have seen some success when people just send a LOR though. You never know.
As for how many IRC, I'm sorry I don't know :( I haven't sent anything out of the country yet.

I want to get some autographs from the Neighbours cast. Could someone tell me if I can use british stamp/IRCs or if they will pay postage. If IRCs, how many will I need? Also, they will have their own photos, wont they?

Thanks in advance