Free or commercial software?
Photoshop is in a class of its own when it comes to image editing. However, it's not cheap.
The most frequently recommended freebie, here on AB, is probably Serif Photoplus 6. (Serif is a fantastic software company. They make much of their older software available free of charge. As most people don't need the 'bells and whistles' of the newer versions, it means that you can get hold of decent commercial software for free):
http://www.freeserifsoftware.com/
Irfanview is also popular. (It's my default picture viewer. I don't use its editing facilities, because I've got Photoshop. However, it's a very flexible program - and totally free):
http://www.irfanview.com/
I've not used Google's freebie, Picasa, but it's obviously popular with contributors to AB:
http://picasa.google.com/
If, by 'banners', you mean large printed pictures (rather than the alternative meaning, related to website design), Posteriza is probably the most popular software for this. It's another freebie, here:
http://www.download.com/Posteriza/3000-2130_4- 10485661.html?tag=lst-0-3
Chris