Photoshop, and the free clone of it Gimp, are great if you want to have very in depth control of your art/images/header. I use Photoshop exclusively. However, it takes days of study to get to even a basic level of Photoshop knowledge so you can turn out nice headers. Other image editors like Paint and Picasa work well but once again, they are image editors and don't specialize in making headers.
If you're interested in an editor that specializes in headers, then I have to second xHeader. You can download it free from xheader . com , it is created by the xSitepro guys and is meant to be very user-friendly, takes moments to download and learn. Very easy to use and you can crank out headers like mad without having to get deep into learning image editing programs. It lays out templates for you, you set sizes etc., import any of your own pics you want, play around with the text effects and fonts. It's very streamlined towards easy headermaking.