That's a very good question. The answer depends on if you're planning to build Pega Mobile App or use Pega web UI in a mobile browser. The former doesn't require Mobile license but the latter does. 

Pega has successful deployments of Pega Mobile apps using 3rd party fingerprint scanners for authentication. 

We will be using a Pega web UI in a mobile browser. Althought we will not use the firgerprint for authentication. 

I don't know if this would be considered the "best" approach but it might be the path of least resistance.

Assuming the image already exists on the Android device doing the scanning, why not build a specialized portal for the users of that device that provides just the functionality need to attach images to specific cases?  You could then create a data page of the cases that are in the proper status for scanning and allow only the Add Attachment functionality for each case.

This all assumes that the Android device has network connectivity.  This way, you could use a browser.  I would hesitate to do this as an offline activity as it would really complicate the case design unnecessarily.

I have definitely not considered all your requirements because I am not privy to them, but this seems like a reasonable option based on what you have presented.  One thing that is not clear to me based on your post is whether the users of the fingerprint scanner are the standard users of the application or if the scanner operators are a handful of users that do only that task.  For example, for a few jobs I have worked, I had to submit fingerprints and I went to a specific office with a specific set of users who only collected fingerprints.  That would be the context of my suggestion.