where relative_path is the relative path from sgallery/includes/ to your image and parameters could be all allowed by phpThumb API GET parameters e.g.
q=80+wp=90+wl=120+hp=120+hl=90+iar=0+bg=373737
Full list of parameters could be found here - [link] However you should disable the security setting from within phpThumb.config.php first.
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