This great piece of software gives you all you need to create detailed graphics for your website, resize images, create banners, add effects and more.
While the final image for your site will be a flash png, or jpg, you can create working file within pixlr than contains layers.
This allows you to create complex layouts that you can easily change such as creating a banner for your website
Pixlr also offers up most of the adjustment options available in Photoshop or camera software that enables you to fix the exposure on images, adjust brightness and contrast, along with some standard filter presets like Sepia, desaturate.
The website offers a lot of other tools as well so be sure to check this awesome online image editing tool that can help you create effective imagess and graphics to enhance your website.
You can use it at http://pixlr.com/editor/