This page will help you create alternate overlays for use with Google Earth. It's true that Google Earth already contains high-resolution satellite data for many cities (and even some non-urban areas); but in parts of the U.S., the USGS black-and-white 1m aerial photos provide better detail, and the USGS topo maps contain information which is not available in GE in any form. For the rest of the world, the images from NASA's MODIS satellites might be of interest, if your data covers a large area; they transmit up-to-date (but low-resolution) images back to earth every day.
If you try to generate an overlay and get an error about a map server being unavailable: my sincere apologies, but it's out of my control.
(If your intent was to convert your tracks or waypoints to KML for use in Google Earth, you'll need to use the Google Earth map input form.)
If you have a newer Garmin GPS, and you want to use this page to create a map overlay to load onto your device, make sure that in the form below, "Ground Overlay type" is set to Static; that should allow your overlays to work properly on your GPS. (A firmware update to your device may be required; see GPSFix.net or Garmin's blog for more details on custom maps.)