It is generally not against copyright law to include a photograph or download of an old painting as an illustration for an article. However, it is important to consider whether the painting is still protected by copyright. If the painting is from before 1923, it is likely in the public domain and therefore not protected by copyright. If the painting is from after 1923, it may still be protected by copyright and using it without permission could potentially violate copyright law.
It is also important to consider whether the use of the photograph or download of the painting falls under the fair use doctrine. The fair use doctrine allows for the use of limited portions of a copyrighted work for purposes such as commentary, criticism, news reporting, and scholarly reports. Whether a particular use of a copyrighted work falls under the fair use doctrine depends on the specific facts of the case, including the purpose and character of the use, the nature of the copyrighted work, the amount and substantiality of the portion used, and the effect of the use on the potential market for or value of the copyrighted work.