[Feature/Improvement] add a Picture for the practices

It is intend to deposit a picture for every exercise?
If the specific exercise is running, the picture can be shown above or below the timer.

It would be very usefully for some practices.

Look at the UI code for the timer screen, likely in lib/screens/. Identify where exercise details are displayed and add an image widget. You'll need to decide on image storage (assets, network) and update the exercise data model. Check existing exercises to see how they are structured and where to add an image field. Test on both Android and iOS simulators.

Scritto dal modello di indicizzazione a partire dal testo della issue.