Hello TheDreamer!
Welcome to Fanmail.biz!

I'm sure you'll love it here! {thumb2}
To answer your questions:
1. I don't live in the US, but I managed to get 90 cent stamps from the US (for the SASE). I'm using the biggest envelope I could find, one of those A4-sized enveloped (the size of a normal sheet of paper). The question is, would one 90 cent stamp be enough to get it back to Sweden, where I live? Or do I need to put 2 stamps?
Unfortunately one 90c stamp wouldn't be enough on a A4 sized envelope from the US to Sweden.

I suggest adding one more 90c stamp and to be on the safe side, another 10c stamp mabey. Then it would add up to $1.90 postage - that's the amount I use on my SASE's (I live in South Africa).

It works perfectly and I never got a RTS!
2. I can't really decide on which picture to send, so I thought I'd send a few... maybe 3? Or 4... Or 5? What are your thoughts on this? Am I decreasing my chances of getting anything signed? Is it rude (or weird) to send that many pics?
I've seen people sending as many as 8 photos to some very busy actors... and he signed them all!

So I think if the actor is a bit unknown, he probably wouldn't mind signing all of your photos! {thumb2} Even if he can't sign them all, he will probably send them back to you so you won't lose them!

Also, I don't think it's rude to send anything under 5 photos, but 8...

So you may send more than one photo! :razz:
If you have any more questiona, please feel free to ask!
Good luck,
Lizanne