Can I store images, videos, PDF's or other formats in Strongbox?

This depends on the underlying file format you choose to use. You can choose to use KeePass or Password Safe formats.

For the KeePass format, yes, you can add any attachment(s) you like to each entry in the database. It is recommended that you keep the file size small in general, particularly if you are using a cloud based database, as synchronizing a large file may take a while.

For Password Safe formats, the answer is No, unfortunately not, since this is not supported currently by the Password Safe file format, these items are not supported, though there are plans to add this capability.