The concern is that any image that you find on the internet is protected by copyright. It is possible that some image is in the public domain, or that it has a license that would allow you to copy such images, but verifying that any such image is "available" is extremely difficult. Depending on the subject, the majority of images of famous people that appear under Google Images are probably not with permission (Google does not host them, they simply collect URLs of images that other people post). If there isn't an explicit license with the image that allows you to download and exploit commercially, you should assume that it is not legal. There are agencies such as Getty Images which have a supply, some of which are hugely expensive to license and some might be affordable (dunno if they have anything for free). There are somewhat better prospects for Mozart, where any painting of him would no longer be protected by copyright. However, the photographs or scans of photographs of the paintings may be protected (museums make money by selling copies of the photographs). Wikimedia is a good source, but they do not guarantee the copyright status of their images. They will give information on usage permission, which may indicate that they have been given permission to make freely available for any use.