The one thing you should NOT do is to upload such images to Airport as so many members and visitors are want to do.
Airport is solely for images of individual airports - buildings, equipment, etc and any images which primarily feature an aircraft which have been wrongly uploaded to Airport will eventually be relocated to Aircraft and listed as 'Unknown' if not identified.
If you wish to upload an image of an aircraft which you have not been able to identify, then you may raise a profile listing the Tail Number (Registration) as Unknown, filling in as much detail as possible in the profile.
However, I have to say that it will then join a considerable number of other such 'Unknowns' and may remain difficult to locate within the database.
For this reason, every effort should be made to identify the airframe at the time the photograph was taken.
You quote The Imperial War Museum, London. If you aware of the lack of identification at the time, ask staff for the identity. If not seen from the position you wish to take the picture from, take another of just the registration. Otherwise seek out the on-line site and if the aircraft is not seen on any list, email them for info. Ask for help in one of our forums, showing the image of the aircraft you wish to identify.
The only advice I can give for Example 2 would be to take several images if possible from different angles. Enlarging these later on a PC may reveal the registration.
Regarding Example 3, there are now ways of identifying overhead commercial aircraft which someone else may be able to help you with.
Uploading an image as 'Unknown' should be carried out only as a last resort as such images are less likely to be viewed.
Malcolm
Admin.