Adobe Photoshop is the most widely used professional image editing program in the industry, and it's used by professional photographers, web designers, and just about any professional who has anything to do with digitally editing images. Besides Photoshop, Adobe has a suite of other software, some of which is specifically for web designers (Illustrator, InDesign, Dreamweaver). See Adobe's website.
(Wow, that sounded like a commercial for Adobe...).
Web designers could draw stuff for websites in Photoshop, using layers, and export it all to Flash, so that a Flash website can be made with it.
PixelMator is an image editing program like Photoshop, ofcourse you can use it to edit images for websites, but it's not really meant as a complete solution for creating websites.
If you're looking for an image editor on the Mac, then
Seashore is also a nice, and free, program you could have a look at. (It's not as sophisticated as Photoshop or PixelMator, but it's good enough for many tasks and it's free, unlike the other two).
