*EDIT: thanks to homeqrown (https://www.deviantart.com/homeqrown) I have an even better solution! You can avoid the background turning to black by doing the following:
go to the line art, right click on it, click 'save image as...', and save it as a PNG file. Then it'll show up as having a white BG.
if for some reason it doesn't work, you can still do the original thing xD
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So, many people told me my lineart's background turned black on MS paint,
and it is because I submit my linearts with transparent backgrounds.
Since a few asked me to put white backgrounds to my lines, and I don't have the time
to turn them all white, I edited a few and submitted them to