OK: difficult to do this without seeing all your code...
2 issues here:
1) you can have a border around an image (the border poperty).
2) you can have a border around an image that is a link (the text-decoration property).
In the code you have written, you have created a class for a link called imgsub, and you have specified that it have no border.
You have then applied that class to your link (correctly), but what about the img tag?
<a href='homeYr11.php' class='imgsub'><img class='imgsub' src='images/yr11.gif' /> </a>
If that doesn't work (and although it should work - it is not neccesarily correct mark-up in relation to the rest of your code), your prob may be elsewhere...