A mirror image is a reversed image of what you look like. If you don't believe it, hold a newspaper or magazine in front of the mirror and try to read it from the mirror image.
You are correct about the image from a cell 'phone camera. That's because cell 'phone cameras have a tiny, inexpensive (made with very cheap glass and no special coatings to eliminate or prevent light glare or aberrations) ultra wide angle lens, and you're standing too close to the lens and experience "wide angle distortion" (also called "wide angle perspective"). If you stand too close to the lens, it makes you seem like your nose, you cheeks and chin are pulled forward and the rest of the face/head are appear smaller by comparison. Here's another thing you may notice, if you take photos of buildings and/or lampposts they will seem to bend inward toward the center of the image. You'll particularly see this wide-angle distortion on "selfies" because the arm of the person taking taking the photo is not long enough.
You can avoid looking misshapen in images taken with cell 'pones by standing no less 5 feet or 6 feet away from the cell 'phone camera lens, and by not taking "selfies." Or have your photo taken with a point-and-shoot camera, or a "Bridge" camera or a DSLR camera. OR you can have a family member or friend stand at different distances and snap off a few photos with your cell 'phone until you see the resulting image that looks fairly good; take notice of the distance and use that as a guide for future photo-taking. At least you'll know some of the limitations of your cell 'phone camera and know what to expect.