in the current functionality a singe image target could only recognize a single image /picture
I think , you can generate instant image targets / generated at runtime/ via script.
You can find an example in the Vuforia Developer library documentation : https://library.vuforia.com/articles/Solution/how-to-create-and-load-targets-in-unity.html
Please, check here the section "How to create Instant Image Targets at Runtime from a file or Texture2D Objects" . This information and also a part of the video ( embed in the article) will provide a guidance for create instant image target. There is shown only the creation of one image Target but this techniques could be scaled to a creation of large list of targets based on input with list of image files.
A second option (but it requires cloud data base /Cloud recognition functionality) is to generated a large list of cloud image targets by API. This is documented in the Vuforia Developer library article :
This article described Rest API calls / methods of VWS: The Vuforia Web Services API enables you to query, upload, and manage images in Cloud Databases and to obtain reports on your targets and databases using a REST based API via HTTP. You can also use VWS to generate printable instances of VuMarks using the VuMark Generation API. You can download examples in Java, PHP and Python which demonstrates ow To use the Vuforia Web Services API. (https://developer.vuforia.com/downloads/samples#downloadModal)The Java and PHP samples show how to manage cloud targets from your own content management system. The Python samples show how to automate the generation of VuMarks and implement the Cloud Recognition Web API.
E.g. a good example which could be used as template to create a list of cloud image targets is the java file : PostNewTarget.java (part of the java examples)
Vuforia Engine Support