A photo is made of millions of pixels with smooth changes in tone. Embroidery is made of stitches, each at least a thread wide, in a limited set of thread colors. So a photo cannot be copied into stitches; it has to be redrawn in a way thread can show.
The digitizer decides what carries the likeness. In a portrait that is usually the eyes, the mouth and the outline of the face and hair. Background clutter, fine texture and subtle tone changes are simplified or left out.
Automatic photo-stitch tools skip that judgment, which is why their results often look muddy or stiff. Our guide Can AI digitize embroidery? explains where automatic tools fall short.
Photo embroidery is not the best route for every photo. If a clean, graphic look would suit you better, an illustrated design often stitches more clearly: see mascot and pet portrait digitizing.