Gimp's Free to download (it's open source). It will enable you to create the kind of gradient you're looking for, but it'll take some practice.
Other then that, you could try haking it in your site's code. I'm not an expert when it comes to this, but what I would try is to for every point drawn, calculate the nearest point on the image itself, get that points colour, calculate a gradient from that colour to the desired edge colour and use the distance to that point to pick the right colour in the gradient.