Is it possible to check dimensions of image before uploading?

You could check them before submitting form:

window.URL = window.URL || window.webkitURL;

$("form").submit( function( e ) {
    var form = this;
    e.preventDefault(); //Stop the submit for now
                                //Replace with your selector to find the file input in your form
    var fileInput = $(this).find("input[type=file]")[0],
        file = fileInput.files && fileInput.files[0];

    if( file ) {
        var img = new Image();

        img.src = window.URL.createObjectURL( file );

        img.onload = function() {
            var width = img.naturalWidth,
                height = img.naturalHeight;

            window.URL.revokeObjectURL( img.src );

            if( width == 400 && height == 300 ) {
                form.submit();
            }
            else {
                //fail
            }
        };
    }
    else { //No file was input or browser doesn't support client side reading
        form.submit();
    }

});

This only works on modern browsers so you still have to check the dimensions on server side. You also can't trust the client so that's another reason you must check them server side anyway.

Yes, HTML5 API supports this.

http://www.w3.org/TR/FileAPI/

var _URL = window.URL || window.webkitURL;

$("#file").change(function(e) {

    var image, file;

    if ((file = this.files[0])) {

        image = new Image();

        image.onload = function() {

            alert("The image width is " +this.width + " and image height is " + this.height);
        };

        image.src = _URL.createObjectURL(file);


    }

});​

DEMO (tested on chrome)