Each time the program downloads a picture, it downloads it from one of my servers. The logs of the servers tell me which image was downloaded, what's the operating system of the user, what version of Coollector he's using, and which skin he has installed.
I parse the logs daily to count how many images were downloaded with each skin. Because a user might use the program heavily and download thousands of images while another one might download only a few hundreds, the number I have isn't the number of persons, but a measure of the usage of the program (images downloaded). I convert the number of images downloaded into percentage to make it more understandable. You can read it as a percentage of activity.