Brush within certain area

Hi, all. I am trying to create a experiment which let the participants draw on some incomplete images. I am wondering if there are some ways which can limit the area they can draw? I want them to draw within the images, but it seems that brush tool can draw anywhere in the screen.

Moreover, I also met the issue of cloneness of lines while drawing slowly. I am wondering if there are some potential ways to help with this issues as well.

Appreciative of any advices and help!