Hello @julienher,
To display images with the checkboxes you should enter images tags as the texts of the choices: <img src="http://www.website.com/image.jpg" />
Concerning to your first question, if you use a checkbox field in your form the users can tick the number of options they want, however, you can configure the field as required to force the users selecting at least one option. But if you want the users must tick a specific number of choices, would be required customize the field’s behavior.
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https://cff.dwbooster.com/customization
Best regards.
