Yes. If you are painting for the purposes of selling the painting then you are by definition trading in it and any profit (you can offset the costs of paints, canvas, etc) would be taxable.
In practice, it's unlikely anyone would notice at the sort of values you are talking about so it's up to your own conscience whether you wish to bother registering. If you have no other significant income then it wouldn't be taxable anyway. If you have a job then it would probably be fully taxable. You can avoid NI on the grounds of de minimis though.