Only a handful of the most recent GPS receivers from Garmin (like the Colorado, Oregon, Dakota) have the ability to display scanned topo maps or aerial imagery. If you have one of these GPS receivers, ExpertGPS can create custom maps that you can download to your GPS and view in the field. Otherwise...
If you don't have a Garmin GPS receiver that supports Custom Maps, your GPS can only display line-based (vector) road maps and contour lines. You'll need to use Garmin MapSource, Magellan MapSend, or Lowrance MapCreate to send vector maps to your GPS. ExpertGPS cannot read or write the .IMG format used by MapSource, or the proprietary MapSend or MapCreate maps.
What you can do, and what many GPS users do to work around the less-detailed maps on their GPS, is to use ExpertGPS in conjunction with the line-based maps on your GPS. Because the vector topo maps on your GPS aren't that detailed, you'll have trouble getting a clear picture of your surroundings on your GPS receiver's small screen. Most ExpertGPS users print out a paper topo map or aerial photo with UTM grid lines, and set their GPS to display coordinates in UTM, WGS84 datum. This makes it very easy to keep track of your position in the field. Use ExpertGPS to plan out your trip, and send waypoints, routes, and tracklogs to your GPS. When you return from the field, ExpertGPS can display your GPS data over USGS topo maps, aerial photos, or street maps.
Don't put the maps directly on your GPS's GARMIN drive. Put them on the removable data card. That way, if for any reason the GPS doesn't like the custom map, you can just remove the data card to start your GPS.
You should also double-click the generated .kmz file and verify that it displays correctly in Google Earth before transferring the kmz to your GPS data card.