Here in wordpress tutorial, How to prevent wordpress images hotlinking but allow google. Steeling the blog images is very major issue in wordpress sites. Sometimes people are just linking the your site images in there site.You can read more about linking in above article.
prevent wordpress images hotlinking but allow google
http://en.wikipedia.org/wiki/Inline_linking
Some sites are using your images and your bandwidth. If you are still want to use other images then use in your server and give credit to there owner sites.
I already wrote about this.
How to protect your images directly accessing through server by apache
Hotlinking’s legality, purely in regard to bandwidth theft:
http://www.phenomenalwomen.com/donteventhinkaboutit/
http://www.yourhtmlsource.com/sitemanagement/bandwidththeft.html
http://www.boogiejack.com/copyband.html
http://www.webprogramming360.com/2011/05/what-is-bandwidth-theft.html