Are you the victim of bandwidth theft? If you have a web site it will be hosted on a server somewhere (usually at a dedicated hosting provider).
Each time a user views a page on your site, the files that make up the page (including pictures) are transferred to the user's PC. Your hosting agreement will allow a limited amount of data to be transferred each month before additional charges apply or, with some suppliers, your site is suspended.
Bandwidth theft (more correctly called hot linking) is where another web site incorporates a direct link to one of your photos. This may not seem important but apart from the copyright implications it can be a very expensive problem.
The practice is commonly a problem with blogs and forums. If they are themselves busy sites, your data transfer limits can be breached and you might find yourself with a large bill for extra data transfer.
At best the practise is distorting your statistics and making you believe that your site is more popular than it really is.
You can spot the practice by looking carefully at your web statistics. An increase in the number of visits without a corresponding increase in the number of page views is an early warning.
When discovered, you might like to change the name of your image which will result in a broken image on the other web site.