What I want to do as following:
use remove.bg tool to remove image (bg.jpg) background, I got a png file (bg_remove.png) with background removed (not perfect).
use gimp to scale bg_remove.png to the same size as bg.jpg.
load bg.jpg to gimp and apply mask from bg_remove.png.
try to edit the mask layer to ajust the not perfect region.
The first 3 steps is good but when I do it in step 4, I can select a region, then file black color to make the region visible, but for some other part i want to hide, when I try to select the region then fill the white color, it does't take effect.