Disclaimer: My answer below does not constitute legal advice, since I am not a lawyer. If you want legal advice, hire a lawyer.
In most cases, actual people themselves cannot be copyrighted. There are some exceptions such as fictional/literary characters. In most cases, it's only images of people which are copyrightable works. Copyright also covers the making of derivatives, such as creating a new work based on an older work that is still under copyright.
The licence on Wikipedia for the image you linked to, claims that it's in the public domain which means the copyright has expired. However, please note that just because Wikipedia claims the image is in the public domain doesn't necessarily mean it is. It's your responsibility to check.
According to the licence page on Wikipedia, the etching is in the National Portrait Gallery in London. So, if you want to check, that's probably the best place to start. Contact them and ask.
Copyright laws also differ by country. Here in the UK, generally speaking, copyright lasts 70 years after the death of the author/creator of the work, although apparently there are some exceptions to that rule.