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.