Before you animate: choose the right still
A video model can only preserve what the still shows clearly. Use a portrait where the face is sharp, the lighting is even, and the costume has a readable silhouette. A busy background gives the model more surfaces to hallucinate motion on, which makes the clip feel unstable.
Hands, thin jewelry, and loose straps are common sources of shimmer. If you have a version of the portrait without them, use it for the animation. You can always add a still image with hands to the final post separately.
