looking for background editor

hi,
I've been making a horrorish type game but i cant seem to find any software to edit a background though. I'm tryng to create quite complicated backgrounds so I'll need all the basic stuff but I especially need one that I can controll opaciaty with. I cant seem to find one anywhere, im using windows 8.1 so paint is out of the question, there isnt any that look legit that I can find. can anyone help me?

http://www.gimp.org

If you have a Mac... http://www.pixelmator.com