To make it more clear, you can't make assets FOR the jam beforehand. If you have assets before the jam, that's good, But you can't make sprites with the mindset of them being specifically for the game jam. So when the jam starts, and you get an idea of "hey, i could use (insert existing thing of yours here) to make things easier!", it's absolutely valid to do so, as long as you're not simply taking an entire pre-existing project and making it your jam submission.
Making an asset for the jam beforehand means, working on the jam game before the game jam starts. Which is not allowed.
However, After the jam starts you can make your assets or use the assets that you have right to use (As they were not made with intentions to be used in the jam). As you dont know what the theme is until the the jam starts, you have no strong idea of your own game, so any pre-bought/made assets has a draw-back of most probably not fitting the theme/feel of your game as you might have intended after having the theme.