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If you find an image of a line, you can add it the same way that you add any other picture.

The line is a vector image. I actually got that made by a friend. I have no idea how to make this myself which is why he made it for me. But the simple version is to find some sort of line that is a vector image and paste it into your posts.

Here are the details and feel free to ask him about it. https://steemit.com/steemit/@valorforfreedom/pjqcae8l