If I understand you correctly, you seem to want to make an image of the white shape, where the black parts are transparent.
I don't normally use XnConvert. I just downloaded it, and was able to find this solution:
First add a Levels action. Pull the white slider all the way to the left to make the image completely white.
Then add a Add mask action. Use the ...
A slightly modified answer, to make it a complete answer.
Install ImageMagick via terminal.
Resize all the images in the folder you want to change, to a sample size, e.g.:
magick mogrify -resize 256x256 *
mogrify -extent 300x300 -gravity Center -fill white *