.content_img:first-child:nth-last-child(2),
	.content_img:first-child:nth-last-child(2) ~.content_img{
		width: 150px!important;
		height: 150px!important;
	};
	
	.content_img:first-child:nth-last-child(n+3),
	.content_img:first-child:nth-last-child(n+3) ~.content_img{
		width: 100px!important;
		height: 100px!important;
	};

上面css选择器 ~ 选择所有同级的元素报错