Here's the applications I use on my Ubuntu 8.04 (Hardy Heron) PC:
Image editor: GIMP
CSS/XHTML editor: gedit (a bit like Notepad for Windows but with code highlighting, and for Linux)
I stopped using WYSIWYG editors when I was 12 because they usually produce disgraceful code and I always ended up fixing the markup. I just figured I should write my own code from scratch, that way I'd learn more (and faster) and I'd produce better sites. I'm glad I did it, otherwise I's probably never have bothered with PHP and never needed to move hosts, therefore never have found x10Hosting
I'd strongly recommend everybody to view the source of their web pages once in a while, once you learn how everything is sorted into tags it gets quite simple. And there's always W3 Schools to help you pick up the basics.
I've seen a few recommendations for VWD Express. It actually isn't bad, but I should point out that any page you create with it is automatically converted to UTF, which is a problem if you're working with PHP cookies, since even if you use the ob_start() and ob_end_flush() functions, the cookies won't work since the interpreter parses the Unicode appended to the very start of the file. Also, it's more oriented around ASP.NET, and doesn't provide syntax highlighting for most languages (because Microsoft wants your custom -_-).
Yes I know what it's called, but it has great PHP highlighting/code hints, and highlights HTML as well. It has tabs and a file browser, a preview window (though I still use refresh in FF & IE), and has buttons/shortcut keys for inserting the useful HTML elements.
If you code in a mixture of php and html/css/js, this is a great editor.
Only problem is, I can't find where I downloaded it from - it might be at: http://www.phpeditor.kit.net/
but that site is not in english...so I don't know!