As of 0.21, MythTV includes a channel icon downloader in mythtv-setup which will automatically download the icons for channels it knows about, or guide you to find icons for channels it doesn't.
One of the best sources of channel icons is http://www.lyngsat-logo.com/ . There, you can browse the available icons and choose those appropriate for your system. However, doing so can be a lot of work (requiring many page hits and a lot of time). Once downloaded you should put the resulting JPEG files into .mythtv/channels/ for the user running the front-end. To add them to a channel you will need to run mythtv-setup and go to the channel editor and specify the icon file name for each channel.