how to override the placeholder icons in AEM 6?

Hi all,

I would like to override the icons of the cq component placeholders.I have found the Icons here:http://localhost:4502/libs/cq/ui/widgets/themes/default.css example: The placeholder icon for the Image Component is difined here

.cq-image-placeholder {background: url("default/placeholders/img.png") no-repeat scroll 50% 50% rgba(0, 0, 0, 0);}

Now I would like to override the img.png with myImg.png. I have expended my the css file content\src\main\content\jcr_root\apps\myProject\components\content\image\clientlibs\image.css of the image Component as follow:

.cq-image-placeholder {background: url("myImg.png") !important;}

but nothing change.

How to override the placeholder icons in http://localhost:4502/libs/cq/ui/widgets/themes/default.css?