Usually what they say is that if your work is "substantially different" from the original work that you started from, you should be fine with distributing it. If any reasonable person could look at your image and say that it bears very little resemblance to the original outline drawing, then it can probably be classed as an original work and therefore the copyright would belong to you. This is of course kind of a gray area since the phrase "substantially different" is a subjective judgement that will vary from person to person. But if you really did put in "a good deal of work" on it, you should be fine.
On the other hand, scanning in the image and importing it directly into Designer with minimal changes and then distributing the resulting PTN file would be a definite copyright violation, of course.
Clear as mud now, I hope.