Using open source to create images for your startup
The web design bar is set pretty low for sucessful startups if you have a look at the Google 1997 Page. You might want to do better and there's a good reference on free CSS Layouts And Templates in Smashing Magazine. If you don't want to use off the rails templates you can go all MySpace and knock something together yourself.
Traditionally you would have had to use something like Photoshop but it's entirely possible to use an open source tool like Inkscape (lack of design talent withstanding).
The only problem with web design is that you are caught between using a standards-based way of doing things or supporting browsers that most people use. Look at this problem with PNG in Windows IE if you're trying to talk yourself out of doing it.
If you need create a logo it's recommended you use a scalable vector graphics (SVG) in order to produce consistent range of image sizes (from buttons to billboards) from one master image. Have a look at the Fedora artwork by Nicu B - all the work is in the public domain - he has SVG images for logos, t-shirts, and even visio-like organisation charts.
Inkscape, as used to create the Fedora artwork, is an open source SVG editor. It has a helpful "Elements of Design" tutorial so you can do your Design 101. The tutorials are all written in SVG so you can play with the images as you learn. If you have an hour sometime there's a Google engEdu talk that's worth watching about SVG in general and using Inkscape below.
Each SVG image is described as an XML file so you can manipulate it with Java or a bunch of other languages. There an ONJava.com article on data-Driven SVG Apps: a rapid development approach. The images may be on the large side but it does make for an interesting prototype. Firefox (and free plugins for IE) can render SVG natively and there is a case for using SVG to create Rich Internet Applications.
Inkscape can export svgs to png. You will probably need to use the open source Gimp if you want to support to gif, jpg or other common image formats.
Google's first attempts at web design were pretty unspectacular and MySpace limitiations haven't held it back. Inkscape lets you be creative with SVG so take Nicu's excellent contribution to the public domain for inspiration and play around.