Fully transparent pixels do not get filled. Semi-transparent pixels get filled at a lower density. Remove the parts of the photo you don't want sewn by making them transparent in your starting 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 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 Google search will provide more programs and instructions on how to remove backgrounds from images and save them as transparent png files.