I use PhotoImpact (c.£50) for all my grpahics creation and image manipulation; it does more than Photoshop Elements for less and can certainly handle all that you are asking. It will take you a couple of months to find all the features. A more basic free alternative is The GIMP if you fancy open-source.
However, you brought up the issue of banner content. I am bound to mention the need to think carefully about how much information/text you present in a graphical format, particularly if you reference your images in your CSS rather than in your markup. If you're anything like me, you'll be keen to do things properly, and that means seperating content and style as much as possible. CSS images cannot have alt text.