Icons that are universal, are consistent. In order to make icon comprehensible, icon should be simple and notable.
According to Wikipedia, icon is a pictogram or ideogram displayed on a computer screen in order to help the user navigate a computer system or mobile device.
To summerize, Icon is a symbol of a particular thing.
Visual legibility is very important in icon. Which means, icon has to be easily recognizable. For this reason, icon is usually made out of images.
Icon also has to be supportive to contents. It takes the viewer only few seconds to decide whether it is worth spending time staying on the website.
So when the viewer visits a website, images, icons are mostly the things that are sticking out and outstanding. Therefore, including some visual interest on the website will draw the viewers.
So, icons should be noticeable and simple for visual legibility.