I must admit that I use a variety of non standard tools so there may be others better suited to answer app specific questions but I will say that Gimp is free and open source. Also Xara is open source for Linux but closed for windows I believe. Not sure if either of these would be suitable for the case you have attached tho I reckon if you got something where the pencil was gray and the grid was light blue, or a different shade of gray even or that you had something else to key on then either Gimp or Xara would probably be able to do something better than my result. Normally this sort of thing can be done quite successfully.