An unaltered image in itself remains in the public domain in all cases.
The exact circumstances of the placement of the image, if it is part of a page in a book with words, or a caption, or even the page number, may be copyrightable. Whether the reuse of that page would be a violation of that copyright, however, depends entirely on the circumstances of that use.
Similarly, the exact circumstances of the “tweaking” of the image may produce a new and copyrightable image.
There is no general rule that can be applied. Everything depends on the particular and exact circumstances of an individual example.
The main case on the issue in the US is apparently Bridgeman Art Library v. Corel Corp., 36 F. Supp. 2d 191 (S.D.N.Y. 1999), in which the court ruled that “slavish copies” of public domain works are not copyrightable.
Only a handful of copyright suits ever make it through the court system. And those are obviously going to be the ones with the oddest points and the most money at stake, so they’re hardly representative.
In the real world, your lawyer will tell you that the cost of a suit is far more than the rights are worth. So you either settle or don’t do it in the first place.
A four-point test for Fair Use is written into the law and the judges I’ve read are meticulous about applying the tests. Even so, when you’re dealing with the edges of the law one person’s interpretation may not be yours.
All four points must be considered. Being commercial is not in and of itself a bar to fair use.
It’s not necessarily public domain just because it is from 1920. Copyright is life of the author plus 70 years. If you had a 30 year old in 1920 (born 1890) copyright the imagine and he died in, say 1970 (age 80) then the copyright will be in effect until 2040.