The better way is using 'background-size'.
.pnx-msg-icon .pnx-icon-msg-warning{
background-image: url("../pics/edit.png");
background-repeat: no-repeat;
background-size: 10px;
width: 10px;
height: 10px;
cursor: pointer;
}
even if your icon dimensions is bigger than 10px it will be 10px.