I you're doing this from javascript this should work:
function chkImg(box)
{
if(box.checked == true)
{
document.getElementById("img3").style.display = "none";
document.getElementById("File3").style.visibility="hidden";
}
}
Make this change to your checkbox in your HTML:
<input type="checkbox" id="CheckBox1" onclick="chkImg(this);" />
You had a funny character in the attribute name "onclick" and the Runat="server" attribute is not needed. Passing "this" to the chkImg function gives you immeadiate access to the checkbox without fetching it via document.getElementById for instance.
Cheers!