The best option for NTO marketers to ensure there is a limit to the number of times a particular image asset can be chosen for a customer is to set Fatigue Rules for the product Asset Classes. Fatigue rules allow you to specify the maximum number of times an asset can be chosen for a customer in a given time period. This ensures that customers do not receive the same asset too often, and that the asset pool is diversified. For more information on setting fatigue rules, please see the documentation here: https://help.salesforce.com/articleView?id=mc_es_dynamic_content_fatigue_rules.htm&type=5.