Must agree with Tax, Adobe Photoshop series is unbeatable. But it takes a little learning to use it well.
For simple purposes such as creating covers, I would suggest trying to look for an older version because of how expensive Adobe products are. Adobe version 5, 6, 7, or CS would probably all serve you equally. Might be hard to find version 5.0 for sale, ebay it...lol
You might also like to look into a software called ACDSee and they were freeware up until version 3 I think, but after that they require to be purchased. They are up to version 6 but version 5 is almost as good as version 6.