Install gimp
https://www.gimp.org/
then open it.
Click on File -> Open As Layers and select the images you have.
Go to Windows -> Layers and make sure the image with the naked girl is below the other image.
Duplicate the dressed image twice and rename the layers "top", "bottom", "dressed", "naked".
Right-click on all the layer names and click on add alpha channel.
Finally, select the layers you need (naked and top or bottom) and click on File -> Export to save it.
Variations of a Dakun Art image #loli