When you play with firebug, the thing you want to look for is the specific container that holds the element you are trying to alter.
In the case of the image, it wasn't the <img> you were needing to alter but rather the div.itemImageBlock that contained it.
In the case of this price, firebug will show you this line:
<span class="PricesalesPrice">$ 1,699.00</span>
Therefore, in your css, you can create a line such as
span.PricesalesPrice {
font-size: 20px;
line-height: 22px;
font-weight: bold;
display:block;
}
and so on...