That really is out of the scope of this support forum. Copyright law is the realm of high priced lawyers, especially for web sites, since they can be viewed internationally, and copyright laws vary by country around the world.
Beyond that, common courtesy says you should ask permission and then give credit on your web site once you have received that permission.
"It depends" is the correct answer. I, for example, would never ask permission before using an image from the section of a corporate web site that offers pictures for Media to use freely. At the other extreme, a professional photographer's work on his own web site or photos of a restorer of antiques' work both are likely to get you in trouble.
Lastly, were someone to complain to your web host that you were displaying copyright material, you could be banned for life from that web host. That is part of their Terms and Conditions.
Some real world examples: some law firm has contracted with a newspaper in Las Vegas to sue every web site that has used text or images from that newspaper. I know someone who was involved in one of these lawsuits for publishing a single article from the paper, complete with a link and credit.
Second, a fellow from Italy restores 1960s era shortwave radio receivers, and he constantly searches the world for anyone who is using his photos of these restorations, and threatens them.