Here are 3 ways to accomplish a high-precision, trace-style outline without the fat-finger effect.
Similar to Kit Grose's excellent answer. A mask gets applied, and you can use brush and eraser to adjust the boundary. The only difference here is, if you need to see the detailed interior of the wound, then the mask works in reverse, i.e. the mask gets applied and you selectively erase the area you need to focus on. This way the selection gets more, and not less visible.
Use a proxy trace pad. This allows you to use arbitrarily small pointers to accomplish the trace, and the user draws using the trace pad. Very fine precision can be made by adjusting the movement ratio between the trace pad and the screen, as well as allowing the user to pinch zoom.
You may wish to add a slider for 'sensitivity' to adjust the tracking ratio.
Since you are tracing a continuous boundary, the user can lift his finger at any time from the trace and once he sets it back down, the trace just continues from its existing position. So you don't run into issues "running offscreen" with the drawing line.
Drag and drop pen to place it, then push and pull the pen around the screen to keep drawing. This allows the user to decide where she wants to place her finger, and the contact point is separated from the tip of the pen so you can see where you're drawing. This approach also has the benefit of allowing the user to lift his finger and rezoom/pan the image before continuing the trace.
You will need to work out how to remove the pin once it's placed... A remove button can do the job easily.
The wireframes are just a really rough sketch and you'll have to add some refinements. But it's enough that conceptually you can get it.
I really like this question because it's about how to push the boundaries of a ubiquitous touch interface while retaining all of the natural swipe/pinch/drag movements that users have become accustomed to.