Keep your lines thick in your illustration. Small lines that show up on your printed image will show up more thinly or not at all on your screen. A monitor has a much lower resolution than ink to a printed page, and makes illustrations harder to view or read. Keep the lines in your illustrations thicker and your audience will be able to see the detail of your illustration more clearly. Your illustration should download quickly.
Speed is still more important than creativity.
No matter how good your illustrations look, visitors will leave your site if they are too big in file size and take too long to download. A single illustration shouldn't be any bigger than necessary. Test out your illustrations at a smaller file size, either by reducing the amount of colors or having them take up less space on your screen. Your illustration should attract attention. Test your illustrations out with people not familiar with your web site.