It is tempting to train a character from the one photo you like best. The trouble is that a single picture only shows the model one version of you: one angle, one light, one expression. Every new image then leans on that single view, and the moment the pose changes, the likeness slips.
Give it more than one angle
A face looks different from the front, from a slight turn and from the side. Including a few angles lets the model learn the shape of your face rather than a flat copy of one photograph, so it can draw you convincingly from wherever the scene needs.
Vary the light
Photos taken in daylight, under warm lamps and in softer evening light each show your features a little differently. Mixing them teaches the model what stays the same about you, which is exactly what you want it to hold on to.
Change the expression
A set made entirely of the same smile will produce the same smile in every scene. A few photos with a relaxed face, a laugh and a more serious look give the model the range to match the mood you ask for.
Keep the background simple
Busy backgrounds can creep into what the model learns. Plain walls or soft, out of focus settings keep the attention on you, so the character is built from your face and not from whatever happened to be behind you.
Quality beats quantity
Six or eight clear, well lit photos work far better than thirty blurry ones. Sharp pictures with your face filling a good part of the frame are what the model can actually learn from, so choose carefully rather than uploading everything.
Check the set before you train
Look at your photos together before you start. If they could all have been taken in the same minute, add a couple that differ. If one looks nothing like the rest, leave it out. A few minutes choosing well saves retraining later.