A abstract archictecture required for a system

Hi. I’d like some high level/abstract advice please about a small application idea I intend to develop…

Basically I want to develop a web based application that allows users to view a random image from Ebay.com or amazon.com. They can only view one image at a time. The user is invited to write a brief one lined opinion about the image, and then submit it, and then they’re shown the next image and so on. The user can end the proocess whenever they want.
The opinion for each user is stored.

That’s it.

I’m not really familiar with web development. Below is my opinion of what I think I need to do…

Have a separate class for users, image, opinion.
Perhaps the user should have a session which allows for uniqueness.
I will store / gather opinions about each image in a database.

I intend to develop this in PHP.

Really what I’m looking for is advice about establishing an architecture for the system. Also is it possible to add web services that display images form these sites?

Any questions please feel free to ask.