This one is going to be short since I'm not a designer. I've been reading "Web Design for Developers" by Brian P. Hogan and he recommends using Illustrator for parts of his book. Since the cost was a bit umm... prohibitive for learning purposes, I started looking around for a suitable alternative. What I found was Inkscape. This is a nice little program for generating SVG (Scalable Vector Graphics) files. You can use it for generating files for say logos or banners on websites. While I won't say it's simple to use (and this probably says more about me than the program), with the tutorials available, you shouldn't have too much problem getting it to work. Additionally, there's a book on it that's reviewed on Slashdot. I haven't read the book yet but do have it on order and if it seems worthwhile, I'll post here about it.
To install ...
sudo apt-get install inkscape
and you should have an inkscape selection Applications/Graphics or at least on Ubuntu.
Once again, I'm not sure how much I'll be able to help, but if you have questions, be sure and let me know.