Marketpress Styling

I've been trying unsuccessfully to modify some of the marketpress styling and make it a little more compatible with my existing styling. I have a few questions....

1) I am trying to override the styling for the images shown here (http://www.sanctuaryatthesuites.co.uk/store/products/) to remove the shadow. I can't seem to override and if I update the actual file (icons.css is what im using at the moment) do I lose the changes in the next update?

2) The checkout page has some truely dreadful old icons on it (http://www.sanctuaryatthesuites.co.uk/store/shopping-cart/checkout/), same as before, can these icons be overriden? The paypal icon appears to be generated by paypal (<img src="https://fpdbs.paypal.com/dynamicimageweb?cmd=_dynamic-image&buttontype=ecmark&locale=en_US" alt="PayPal">:wink: when I know for a fact there are much, much prettier icons that could be used. Where and how would I update these without losing the changes during an update?

Thanks!!!

Julian