Remove all attributes

A simple method that doesn't require JQuery:

while(elem.attributes.length > 0)
    elem.removeAttribute(elem.attributes[0].name);

One-liner, no jQuery needed:

[...elem.attributes].forEach(attr => elem.removeAttribute(attr.name));

Update: the previous method works in IE8 but not in IE8 compatibility mode and previous versions of IE. So here is a version that does and uses jQuery to remove the attributes as it does a better job of it:

$("img").each(function() {
  // first copy the attributes to remove
  // if we don't do this it causes problems
  // iterating over the array we're removing
  // elements from
  var attributes = $.map(this.attributes, function(item) {
    return item.name;
  });

  // now use jQuery to remove the attributes
  var img = $(this);
  $.each(attributes, function(i, item) {
    img.removeAttr(item);
  });
});

Of course you could make a plug-in out of it:

jQuery.fn.removeAttributes = function() {
  return this.each(function() {
    var attributes = $.map(this.attributes, function(item) {
      return item.name;
    });
    var img = $(this);
    $.each(attributes, function(i, item) {
    img.removeAttr(item);
    });
  });
}

and then do:

$("img").removeAttributes();

Instead of creating a new jQuery.fn.removeAttributes (demonstrated in the accepted answer) you can extend jQuery's existing .removeAttr() method making it accept zero parameters to remove all attributes from each element in a set:

var removeAttr = jQuery.fn.removeAttr;
jQuery.fn.removeAttr = function() {

  if (!arguments.length) {
    this.each(function() {

      // Looping attributes array in reverse direction
      // to avoid skipping items due to the changing length
      // when removing them on every iteration.
      for (var i = this.attributes.length -1; i >= 0 ; i--) {
        jQuery(this).removeAttr(this.attributes[i].name);
      }
    });

    return this;
  }

  return removeAttr.apply(this, arguments);
};

Now you can call .removeAttr() without parameters to remove all attributes from the element:

$('img').removeAttr();