What do you mean?
If you have uploaded the image to Firebase Storage or Firebase Firestore then you just need to the the URL of the image and save it either directly in the Thunkable designer (a good example is having an image component in the screen and using the URL as the name of the image in the component).
If, in the other hand, you want it dynamic such as for profile display then store the URL from the image in Realtime DB and retrieve as required by the app.