The simple answer is that there is no simple answer, because this is a very complicated area, often with no clear answers, and where any answer will likely vary from one jurisdiction to another.
The first question is whether there is any copyright in the original document, which typically means assessing whether there is anything original or creative about it or whether it is just a set of facts. Some jurisdictions may allow some form of sweat of the brow based copyright claim however, even for largely fact based documents.
Then you have to consider whether there is a new copyright in the image, which is an area with little good case law to provide guidance. The commonly quoted case is Bridgeman v Corel but that are a lot of issues with that case, not least that it was decided by a US judge but based on his interpretation of UK law, an interpretation that I understand UK lawyers often disagree with. On appeal it was decided that the judgement should have been decided using US law, but was upheld on that basis, so in the US it may be hard to argue for new rights in a photographic reproduction.
Then there is the issue of working our when each copyright may, or may not, have expired. Not too hard for Crown Copyright in the UK but much harder for documents created by individuals or corporations.
The issue of an indexed database that you refer to is mostly only an issue in the EU which has the concept of Database Rights and in the US is unlikely to apply, following the doctrine of Feist v Rural, at least for a simple alphabetic arrangement.
Of course if you are downloading the image from a commercial site then you may well be restricted beyond any copyright restriction by the contractual terms you agreed to when signing up.
Finally, I Am Not A Laywer, so take everything I say as worth exactly what you paid for it...