Give each input a name attribute. Only the clicked input's name attribute will be sent to the server.
And then
Edit: Changed value attributes to alt. Not sure this is the best approach for image buttons though, any particular reason you don't want to use input[type=image]?
Edit: Since this keeps getting upvotes I went ahead and changed the weird alt/value code to real submit inputs. I believe the original question asked for some sort of image buttons but there are so much better ways to achieve that nowadays instead of using input[type=image].