Direct linking is one of the most annoying things ever! To prevent people from doing so, copy the code below into your text editor(Ex:Notepad):
Replace yourdomain.com with your domain's url. Then save your file and name it .htaccess. Make sure theres no .txt or anything else after your file's name or it wont work!
Now upload it to your main directory, and thats it =)! As easy as 1,2,3!^^