First, click on "Filter / PhotoMasque", then click on the small opening file
folder. Masks can be made from the most common formats recognized (JPG,
GIF, png, BMP, TGA, PSP, PSD). After selectiong the mask, a preview is
shown in the corresponding window. Your selection will be preserved for the
next time you use PhotoMasque. It's a good idea at this point to set the
orientation of the mask according to the orientation of the image by using the
rotation button, which is below the preview.
This will minimize mask deformations and optimize the results.