[What's the best practice for storing pictures?]

Hello. I'm working on a website with an editor that allows image uploading. Ideally I want to be able to develop a framework for this and use it in later projects. What I have in mind is this:
 

• Database has an images table
• Entries contain these fields: id, filename, and a short description
• Images are referred to by their ids in other parts of the application.

That part seems simple to me, but now there are two details I need to determine: the file name, and storing the images.
Big sites like Facebook - as far as I know - parse uploads and store them all in the same format. I can see the huge security benefit there. What are the best ways of doing that? As for the file name, I know PHP has a function to generate a file with a unique name. Is there any benefit to doing that over using the id? (i.e. 1.jpg, 2.png, 3.jpg, etc)