The hidden links are not those that are present on the webpage to be viewed but still they are always set in an image file and can be seen through by using hyperlink. When the image gets clicked by someone it moves to some other webpage. In such a case the Google search results of that website gets hampered.
Yes, it is harmful for your website as it is a part of Black Hat SEO. Which is an illegal way of promoting your website. So better you avoid practicing such techniques.
The hidden links are not those that are present on the webpage to be viewed but still they are always set in an image file and can be seen through by using hyperlink. When the image gets clicked by someone it moves to some other webpage. In such a case the Google search results of that website gets hampered.