Your code
table.tablesorter thead tr .header {
background-image: url(Content/images/bg.png);
background-repeat: no-repeat;
background-position: center right;
cursor: pointer;
}
Corrected code
table.tablesorter thead tr .header {
background-image: url(~/Content/images/bg.png);
background-repeat: no-repeat;
background-position: center right;
cursor: pointer;
}