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<div class="imageBox">  
	<div class="imageMain"><img src="<?php the_field('car_image') ;  ?>"></div>
	<div class="showme"><img src="<?php the_field('car_image_hover') ; ?>"></div>			
</div>  


<style>
.showme{   
	display: none;      
}

.imageBox:hover .showme{ 
	display : block;
}

.imageBox:hover .imageMain{
	display : none;
}
  
</style>