Custom image "pick image..." button not working anymore

  The problem is : 

 The option under Inactive Windows tab, the combo box option "the darkened area should be a custom image" displays a "Pick Image" button, which worked 2 times, once I tried a 3rd image it just stopped working, it displays nothing on the background and doesn't lets me choose another image even if the button is still enabled.

 Requesting : Any way to reset/restore settings (tried to find it on the registry but failed).  

 Using WindowFX 5 (updated) / Windows 8.1 Pro

 Thank you.