The most powerful of the browsers available today is Netscape, hands down. So check you page in a few other browsers, just to be sure that the core of your information and the gist of your style is translating to as many surfers as possible. Almost every Web Page contains some text. It's a good idea to have at least your basic information in text form, because some browsers only display text. Attributes and formatting will be taken care of in the HTML, and the font is only controllable at the browser level, which means beside the size, you have almost no control over it as an author.
Most people like to have at least one image in their web site.
There are advantages and disadvantages to each format. This can be a big plus when dealing with large images. Anything you can do to decrease the transfer time of an image will be greatly appreciated by the surfer. There are many programs which allow you to manipulate GIFs and JPEGs. LViewPro for Windows is my favorite, and I hear raves about Graphic Converter on the Macintosh side of things.