What chairboy said
Let's imagine your index.php file is in your root folder (usualy /www in ftp) and your pictures in the /images/ folder
You can link to this way:

or

// This you can use if your html/php file is in a folder below your root
If you're sure you won't move your files to another domain, you can use a full path
Catch me on MSN today.. it's friday so I can goof around :)