how to draw a line with your finger

Hello! I'm new in Unity and have a question:

I have image with dash line (- - - - - - - - -) - contours of the object. I touch finger on this contours and line become full. How can i do this?

I think to make one image with dash line, one image with full line. Smash images to little areas and then check the touch coordinates and change "image1 touch area" to "image2 touch area". It'is correct? how to do it?