problem with cache clearance

can any one help me.
i created one application to track the people opening the mails which is
send by me from my application. but one problem which became an
obstacle in my path is when any use say tony opens that mail once
his count in db is one,but his count will not increment when he go
back to his inbox and click the mail again.for that he had to
refresh the inbox, means had to press F5 to clear the
cache.actually inorder to track the open users i am passing an image to
the controller from view page

is there any methode to avoid this?

or any method to clear the cache.please help me

Thanks;
Tony