This question already has an answer here:
Imagine I have some image, $I$, and I'd like to define a polygon by sequentially clicking on the image to define vertices $(v_1, ...) \in V$, where vertices $v_i$ and $v_j$ share an edge if $j = (i+1)$. I'd like to end the vertex selection process by right clicking, or hitting something on the keypad - really anything that's convenient. I suppose I'd like the $v_i$ to be specified in terms of image coordinates vs. image indices (if possible).
Is there a straightforward way to do this in Mathematica 9.0?