Currently there's no way to make image fields required.
I wrote the following JavaScript but it's definitely not nice:
$("#view_18 input[type=submit]")
.on("click", function() {
if (!$('#field_28_upload').val() && !$.trim($('#kn-input-field_28 .kn-asset-current').text())) {
alert("Please load a photo!");
return false;
}
});