Sketch Picture is used to create a Sketch that uses photographic reference to generate your Entities
Menu Bar→Tools→Sketch Tools→Sketch Image...
Enter Sketch Environment→Sketch Image→Select File (jpg, png etc.)→Locate (use a logical point as the origin)→Scale
The Scale Handle is used for resizing the image to represent an accurate dimension
If you wanted to insert an image of yourself, you'd take a front facing picture and insert it as above, then use the scale handle to extend from the top of your head to the bottom of your feet, you will then be prompted for a value which would be your height and the image would now be "accurate"
Hide/Show the sketch containing the Image as required
Add them to the other Default Workplanes and align as best you can to get three views of the object you're modeling
Click the image below would benefit from utilize Sketch Images