Using the USGS Seamless Server
From X-Plane Wiki
This article describes how to the United States Geological Survey's server to download orthophotos for use in airport scenery. This data is available for the US only.
To begin, go to seamless.usgs.gov. Click "Seamless Viewer" as seen in the screenshot below.
Click "Seamless Viewer"
In the toolbar on the left side of the window, click the "Zoom to a Point" tool (labeled "XY"), as seen in the image below.
Selecting the Zoom to a Point tool
A dialog box will appear. Input the latitude and longitude coordinates of your airport, then click OK.
Inputting the coordinates for KOJC
After zooming a bit, you should be able to see your airport. Our example airport, KOJC, is clearly visible in the image below.
Zoomed to the KOJC airport
Select all of the orthoimagery that you are interested in from the right side of the window. In our case, the "KS - Johnson County (Mar 2008)" images are the highest quality, so that's what we will download.
Select the "Define Rectangular Download Area" tool from the toolbar, then click and drag it to select the whole of your airport, as is done in the image below. It's better to err on the side of downloading too much than downloading too little.
Selecting the download area
With the area selected, go to the Download tab to the right of the map. Un-check the elevation data (as seen in the image below), then select whatever orthoimagery you want.
Un-checking the elevation data
In the window that appears, select Download, and you're files will soon be ready to use.