element id on mouse over

In jQuery:

$(input).mouseover(function()
{
   var showID = $(this).attr("ID");
   alert(showID);
});

check this

function getid(obj) {
  alert(obj.id);
}

<input style="margin: 8px 4px 4px; width:142px; height:117px;" type="image" id="img" src="images2.jpg" onmouseover="getid(this);" />

The value of intrinsic event attributes is the function body. What you have is the same as:

onmouseover = function () {
    getid();
}

When you call a function without an object, it is the same as window.thefunction(). So you are calling window.getid() so this (inside the getid function) is the window object.

If you really want to use intrinsic event attributes (hint: don't), then you have to be explicit about what this is.

onmouseover="getid.call(this)"

However, then:

var e = document.getElementById(this);

… is nonsense because this is the element and not the element's id.

You could get the id property from this, and use that to look up the element, but that would be silly as you can simply:

var e = this;