What we all know happens when attempting to use lazily-made masks:
Here I'm going to explain in detail about fixing these lazily-made masks:
After opening a broken mask, you must change the mode to RGB before making any changes to the image:
Select "Magic spell" and set "Adding" mode. So now you'll be able to click on white zones which means "transparent":
When all the white zones have been selected, click the "Delete" key to fill them with "real" white color (must be #FFFFFF):
Do inverse of selection:
After inversing, DON'T forget to swap foreground and background colors! (Now black is needed as the 'background')
Now when you've done all this, press the "Delete" key and your mask will be done!
Now you could switch back to indexed color mode (which is needed for GIFs)
The image must be Black&White or gray-grade
After you're done with all of this, you have to overwrite the source image: (File->Overwrite "xxx-YYm.gif" or "Export to..." and save them as GIF)
Sprite-designers, please don't make lazily-made masks next time!
And save files as the real .GIF, not .BMP renamed with the .GIF extension