I am back.
I have tried your suggestion but it doesn't seem to work.
I went to system/configuration/display/css/frontend css file/ and I have edited the default css, resulting in the creation of frontend_custom1.css. I have simply added to the end of the file the following code:
.hikashop_html_add_to_cart_link{
display: block;
background-image: url('http://www.mysite.com/images/stories/ic-cos.png');
text-indent: -9999px;
}
Unfortunately, no change to the custom html button appearance.
Any suggestions? Did I do something wrong?