If I were to use an image from
Modern Guns and Mortars Adopted in the United States Land Service, Their Carriages, Projectiles, Fuzes and Sights which was published in 1895 by the Government Printing Office, I should not be violating anyone's copyright since it has expired. Similarly, drawings made by old artists of old stuff, even though reproduced in a more modern publication, should be public domain.
Now I am no copyright attorney so this isn't legal advice but we have to apply some common sense (I know, it is a legal issue but bear with me) here or we will lose the main feature of the internet, the ability to link to other sources of information so one can further research a subject if he cares to. A link to the source should be a satisfactory way of attribution.
Since we are not usually displaying fine art here, a simple solution is to take an image editor and add a line in an inconspicuous part of the picture recording its source. Here is an example:
This was done with a free image editor called
Irfanview. Also, the image can be clicked to go to the source site. The most difficult thing to do is going to be making this a habit.
