The Web can hold text, but it can also hold graphics, audio, video, and many more media. This is because the references from the HTML in a Web server can be to files of other data types, and because the returned data from a server to a client uses the MIME Content-type facility to specify the type of the contents of the rest of the data. In the example above, the return type was text, and specifically HTML. It could have been postscript, or a compressed program binary, or MPEG video, or PCM audio or just about anything you can imagine being stored on a computer.
In the absence of super-highway speed links, clients are often configured to only retrieve text contents by default. The protocol can defer loading images for example.
Typically, servers store images in GIF format, although anything else that can be described as a mime-type that the client is configured with a viewer for could be used.