To use Audiotap, you have to download their "mediatap" software on your Mac or PC (thanks James). It uses the information from iTunes, like playlists and podcasts, and makes everything available over the internet.
Once you choose the Artist, if you have images on, you will see a screen similar to the pic below:
If you are looking for a cheap alternative ($3.99 at the time I wrote this) to iTunes Match that can handle more than 256 kbps files and you just want to stream files without downloading, Audiotap should fit the bill.