Renders? go get Apophysis. Banners? Photoshop/Gimp (depending on if you want to really pursue gfx or just get a quick fix)
Its a alternative for people who dont have photoshop and choose to abide by the law. No, its freeware and photoshop has more features.
Gimp is the same thing as Photoshop-to an extent. It can get you almost to where intermediate-level Photoshop users are for free, but it takes some work, and there aren't may tutorials for TheGimp on the internet. My best solution is to get Photoshop 7 off if ebay, its only around $100 and most resources out there are made for Photoshop 7 and above.