A new file format is impractical, It would have to be readable by the html since it wouldnt be stored on the server and the php couldnt "decode" it. Unless you have a program that will decrypt them from the file as and when you need them and communicate with the server so it knows when to decrypt it.
A lot of work all in all.
And Orlando, in time you might relise that Mur cares very very deeply about spoilers, And your proposal to track where you have visited and what you have trained and then allow you to download those images? Firstly thats another load of data that needs to be accessed and saved and secondarily Mur doesnt want you to be able to save the images, Hence the image protection.
Im sure Mur knows how to improve serverload, And im sure the community could give some really good ideas. But the downloading of the images pales in comparision to the queries, calculations and general php processing that is done.