Fully transparent pixels do not get filled. Semi-transparent pixels get filled at a lower density.

Remove the photo parts you don't want to be sewn by making them transparent in your photo.

How do I make parts of the photo I don't want transparent?

This site has simple editing tools, and If the parts of your photo you want to remove is a solid color, you can use a flood eraser or color eraser tools to remove areas. If the area is more complex, you would need to use a photo editor like GIMP(free), Photoshop(subscription), or Pixlr(free) to remove the parts you don’t want 

A web search will provide more programs and instructions on removing backgrounds from images and saving them as transparent png files.

 

Resources:

removebg - Remove Image Backgrounds automatically

GIMP - GNU Image Manipulation Program

Pixlr - Online Photo Editor