You can use javascript but if the client has javascript disabled this won't work but if you keep your image on click then it's kind of like a fail safe. I don't know what key you want to use but i'll use the enter/return key.
Code:
document.onkeyup = KeyPress;
function KeyPress(e)
{
var KeyID = (window.event) ? event.keyCode : e.keyCode;
switch(KeyID)
{
case 13:
window.open('your popup link here');
break;
case 10:
window.open('your popup link here');
break;
}
}
I have two cases here because i'm checking for the enter/return key (two different keys) You can use any keycode there you want. Hope this at least helps you and gets you going in the right direction.