PNGs transparency isn't supported in IE6... So you gotta use faux transparancy's for that.

Photoshop - a program made to manipulate photos. Very useful program and very popular in web development.

Illustrator - a program made to create images. Vector image creation, great for logo's, custom text, and print.

Fireworks - OLD: Photoshop knockoff. NEW: A website layout manager. It's made to actually build the layout of a website and then export it to Dweamweaver / Flash. It has a lot of powerful features but its not as easy to use are PS IMO.

HTH