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.
To configure icon downloading in mythtv-setup:
- run "mythtv-setup"
- select "Channel Editor"
- click on "Icon Download"
- select either "Download all icons.." or "Rescan for missing icons.."
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.