1) Can you add public domain photographs to your ebook (the ones that are free to use even for commercial uses)?
2) Are there any nasty repercussions to this?
Basically, I'm planning to release an essay on KDP and wanted to illustrate it with some of the many relevant, public domain (in other words, very old) pictures. I just want a second opinion before I go ahead because I haven't used photos like this before.
Be sure to document where you got the photos and what information you have about their copyright or license status. If Amazon questions the use of the images, you had better be able to show them you have the right to use them.
You are using the term 'public domain' in reference to old photos. They very well may be in the public domain, but lots of people throw that term around without understanding it, so I want to be clear about the legal meaning: Public domain means that the work is not covered by copyright. Often that is because the copyright is expired, but sometimes it means that a work was never eligible for copyright. In the case of photos, one example of works not eligible for copyright could be photos taken by Federal government employees in the performance of their job duties.
Just because a photo is 'old' does not mean it is public domain. Just because a photo is online does not mean it is public domain. Just because you saw someone else use it or claim that it is public domain does not mean that it is public domain. Verify the legal status before using a photo. The penalties can be significant.
There are other issues related to the use of photos, such as the right of publicity. That probably won't apply, but if you are using photos with recognizable people it could be an issue.