i Am new to selenium webdriver and I want to automate a webpage related to my project. I am currently working on login page, however I facing problem with the login button which I am confused which locator to use. The login button is below:

a href="javascript:LoginSubmit('Log In')">
img border="0" src="/opensso/login_images/button_enter.gif"
alt="enter / entrez"
Can you guys please help me out how to use the above with xPath or anything related to JavaScript?

Sorry if I am missing anything, as I am very new to this.