*EDIT: thanks to *JocastaTheWeird 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 my scrapbook.

So, if anyone has this kind of trouble, there are some white background linearts in my scraps.
If you wish for me to edit any other one tell me and I'll try to do it by the next day
thank you for your attention,