Well, I did a bit of muddling myself and stumbled through the process.
The following steps are described in more detail in the Topo7 user guide, found under Topo7 program help button. The user guide button directs you to a DeLorme web page, where you can download the user guide web page to your computer. See (pdf) pages 48-52 “Downloading Imagery” and pages 42-46 “Creating a Map Package.” (Printed pages 38-42 & 32-36.)
Transferring a map to the PN40 is essentially a two step process: First you have to download the USGS or aerial photo map, then you use the downloaded map to create a “map package” that you transfer to the PN40.
The 2nd reference in Siskiyou’s note shows how to download the USGS & aerial maps. DeLorme calls the aerial & USGS maps that you put on the PN40 “imagery.” You have to link to DeLorme’s web site via the Netlink tab at the bottom of the Topo7 map display on your computer. After you select what kind of imagery you want, you get a grid superimposed on the map. Select the areas that you want to convert to an image by left-clicking on grid rectangles. You can select rows or even rectangles by left-clicking and dragging the mouse. After you download the images, you can view them in the left side map in the Topo7 display. Purchasing the images is easier if you have one of those annual subscriptions; it’s well worth it if you download lots of images.
Even though you’ve downloaded the images to your computer, you’re not quite done yet. Best I can tell, you can’t just drag the image files from your computer to the PN40. At least I couldn’t; perhaps I didn't drag the right ones over.
Anyways, you have to go to the "Handheld Export" tab on that bottom panel of Topo7. If you’re viewing the imagery you’ve just downloaded, you can go directly to selecting the portions of the map to save. Click on the “Select/Edit” button, and a grid will display again. This time you get to choose the size grid for selecting map areas. Hint: the smaller the grid you choose, the closer you may have to zoom in before you can actually see it. As above, you click on the grid rectangles you want to save (they turn red). If you click a wrong grid, just click it again to deselect it.
After you’ve gotten all your grids, be sure give your map package a name in the left side of the Exchange window (where it says “export map”), then hit the “Save” button. Your map name appears in the lower panel on the right.
In the home stretch now … Click on the “Exchange” button on the right side of the Handheld Exchange window, and a pop up appears. At the top of the pop up, select from the drop down menu what you what to send it to. Either the PN40 or an SD card (the card can be in either a reader or the PN40); either has to be connected to your computer via a USB port). In the left panel, your newly named map is listed. Select it, then hit “Send” (arrow pointed to the right), and Topo7 transfers the map to the PN40 or its SD card.
And that’s it. From now on, whenever you’re in the area of the imagery, it will show up on the PN40 screen in place of the Topo7 map.
Well, that’s the muddled explanation that came out of all my muddling. It won’t make any sense unless you’re sitting at your computer with Topo7 running. Again, the user’s guide for Topo7 is much better than my paltry tricks and tips above. The best thing to do would be to fire it up and run through tutorials (pages mentioned above) that the guide offers.
