Nielsen Norman Group analysis by Kara Pernice of what makes some icons bad and others good, in a nutshell:

Things that make icons bad:

• Image has an established, different meaning
• Reference not sufficiently obvious
• Poor image quality
• Repeated for every item in a list of similar items
• Not comprehensible standing alone, only as part of icon set

Costs of bad icons:

• Labor to create
• Labor to code into pages and test
• Screen space taken up
• Visual noise impacting users

Benefits of good icons:

• Can convey meaning to users faster than text
• Easier to find than text
• Bigger tap target for mobile devices
• Reinforce brand style

=======================================================

Here are some icons I found on tourism sites that to me are examples of bad ones. Because they give me no clue as to what lies behind them. Does your website have any icons like these? If so, better change.