Hi Chrissyo
Although it is possible to detect Galileo Regio on Ganymede, a large dark central spot can also appear if too much unsharp masking is used. To find out if you have photographed surface detail, or simply have an unsharp mask artefect, you have to check the JPL simulator to see if Galileo Regio was visible when you photogaphed Ganymede. Another, more labour-intensive way, is to see if the detail changes shape over the course of several nights. The Galilean moons are rotationally locked to Jupiter, so the moon will appear to rotate slowly over several nights as it orbits Jupiter.
I have some notes here also,
http://astroimg.org/galilean.html
Zac Pujic
Brisbane, Australia
http://astroimg.org