I am not familiar with SketchBook, but since it is pixel based you may want to also look at a vector-based drawing application such as Inkscape (which is free.)

I am not familiar with SketchBook, but since it is pixel based you may want to also look at a vector-based drawing application such as Inkscape (which is free.) Inkscape is also able to save files as SVG which may be of interest to you since you are also familiar with HTML and CSS. The best approach really depends on how you want to draw, whether you prefer to hand-draw with tablet and stylus, or process/tweak/collage photos, or write code to generate graphics. Regardless of the drawing approach, I recommend getting your preferred drawing pipeline worked out with existing tools, noting the shortcomings (or steps that take too long) and then coding up your own solution if you see value in that.