No color picker in this version since you can load any external palete colors to be used. The drag&drop image should work. Also clicking in the image two times place it to the center ¿?.
Using zoom changes how the image looks because all the effects, palette and shaders are applied to the actual image dimesions.
About th C64 mmm works correct to me ¿? What is the color needs to be added and which color needs to be removed ¿?