All of the map-based visual views (with the exception of the Cylindrical (Google Maps) view and WebGL-based views) within Tracker use Earth images from NASA, which can be easily replaced with your own images.
The default images can be found in the Images folder of your Tracker install. This will likely be one of:
C:\Program Files\Sumus Technology Limited\Tracker\Images
C:\Program Files (x86)\Sumus Technology Limited\Tracker\Images
unless you specified your own custom install location. The images you might want to replace in this folder are:
EarthDay.jpg - day view of Earth (2048×1024)
EarthNight.jpg - night view of Earth (2048×1024)
EarthMonochrome.bmp - used to shade the land and seas (2048×1024)
Make sure your replacement images meet the following criteria:
their size should be 2n+1 pixels in width by 2n pixels in height, where n is some positive integer
their projection should be geographic (also called plate carrée) centered on 0° longitude and 0° latitude
In addition, the EarthMonochrome.bmp image must be a monochrome 1 bpp (bit-per-pixel) bitmap.
Simply replace the existing images with your own, for which administrator privileges may be necessary, and Tracker will use the updated images when it is next started.
A good source for such images is the NASA Earth Observations (NEO) website.