function imgCheck(aForm) {
	if ((aForm.bild.value.length == 0) ||
		(aForm.bild.value == null)) {
		return true;
	}
	else {
		alert("nä.");
		return false;
	}
}

<form action="" method="post" enctype="multipart/form-data" onsubmit="imgCheck(this)">
	<input type="file" name="bild" size="25" />
	<input type="annat mög" />
	<input type="submit" value="Lägg till" />
</form>