Choosing a pleasant 'happy snap' of the subject is the beginning, not the end of the process.
By the way, the subject limits the use of her image to this post only.
Head and shoulders is often more flattering than waist up or three quarter. Especially if you are cutting someone out of a larger photo.
Less detail makes for a more timeless image - you notice the person, not the unfortunate mid-noughties fashion choices, or the incriminating evidence of where the photo was taken.
You can choose an oval, which keeps the visual focus even tighter.
An oval can create a classic, traditional or retro feel. It depends on the original photo.
There are other frame shapes but oval/circle or rectangle/square are classic for a reason. A star-shaped cut out, for example, can look too-cute very easily.
If you're feeling brave, tinker with the mid-tones to get the right amount of detail. You don't want to emphasise every spot or wrinkle, but neither do you want the face to lose definition or character.