I've added transparency to images before, but I am wary of the limits of this. The best transparency I do removes a slight edge, although there are no glaring artifacts if you put it in a black background.
Like these
Transparent
Not transparent
But I wouldn't like to revert since the top one has its uses and it doesn't have the common artifacts that is often present when other people try to remove the white background. So, I call it a sacrifice.
But yeah, about this whole jpg-png, it's extremely vague because we often replace low-quality .jpg screenshots with .png screenshots, but I suppose we can save the .pngs as .jpgs to save space, but is it worth it? Also, you have to consider that .png is the most desirable format for sprites, moreso than .jpg, so all .jpg sprites ideally need to be replaced with .png.
So in the end, maybe you can write about higher quality reuploads being fine to avoid misunderstanding.