Can I use a picture of two actors on my book to show readers where the characters came from?

The simple answer is no. There are a variety of reasons that this is not permissible. Number one, the photographer and sometimes the model have copyrights to the image you wish to appropriate. Just because it is on the internet does not mean that it can be used without first obtaining licensing rights. One major exception to this rule is that of fair use. You may use the copyrighted material of another if it is being used for news, education, parody, etc. Additionally, if you based your characters on the lives of actual people you must first obtain life rights from them. Consequently, your underlying book may violate the people's rights whom you based the characters upon.

No, you can not use a picture of two actors on your book. You must have permission to use the image of another person for commercial purposes.

If the photographs were taken prior to about 1923, they are in the public domain and you may use them. If they were taken after that, they are probably still protected by copyright law and you will need to get permission from the copyright owner to use them.