Hi,
It's pretty simple. Instead of the CSS Nicolas provided here above, just use this:
.hikashop_products_listing .hikashop_product_custom_product_hashrate_line dd.hikashop_product_custom_value:before {
background: url('url of the image') no-repeat center center / 10px auto;
display: inline-block;
width: 10px;
height: 10px;
margin-right: 5px;
content: "";
}
As previously, replace "hashrate" with the column name of your custom field.
And, of course, you may want to replace the 10px and 5px values with your own.