Popims Animator proposes two methods to create transparencies, the definition of a transparency colour, or the definition of a transparency zone.
To use a transparent color, choose the "Color Key " option, and simply define a transparency color by clicking in a zone to be made transparent, and adjust with the "tolerance" selector.
Otherwise, indicate if you wish to define as transparent the Inside selection or on the contrary the Outside selection , and create the transparency limit as indicated below.