Jquery Ajax issue submitting with image

Jquery Ajax issue submitting with image

I have the following ajax code:

$(document).ready(function () {
        $('body').on('click', '.addtocart', function(){
            // Get the form data. This serializes the entire form. pritty easy huh!
            var form = new FormData($('#myform')[0]);

            // Make the ajax call
            $.ajax({
                url: 'test.php',
                type: 'POST',
                xhr: function() {
                    var myXhr = $.ajaxSettings.xhr();
                    if(myXhr.upload){
                        myXhr.upload.addEventListener('progress',progress, false);
                    }
                    return myXhr;
                },
                //add beforesend handler to validate or something
                //beforeSend: functionname,
                success: function (res) {
                    $('#content_here_please').html(res);
                },
                //add error handler for when a error occurs if you want!
                //error: errorfunction,
                data: form,
                // this is the important stuf you need to overide the usual post behavior
                cache: false,
                contentType: false,
                processData: false
            });
        });
    });

    // Yes outside of the .ready space becouse this is a function not an event listner!
    function progress(e){
        if(e.lengthComputable){
            //this makes a nice fancy progress bar
            $('progress').attr({value:e.loaded,max:e.total});
        }
    }

if i submit this data using <input type="button" name="submit" class="addtocart" />
I get the desired result. however if i use <input type="image" name="submit" class="addtocart" /> to submit the page refreshes with get variables (as it would without the jquery) any ideas