table.wp-list-table .column-thumbnail {
    width: 50px;
    text-align: center;
    white-space: nowrap;
}

table.wp-list-table .column-thumbnail .gsf-columns-icon {
    font-size: 15px;
    width: 15px;
    height: 15px;
}

table.wp-list-table td.column-thumbnail img {
    margin: 0;
    width: auto;
    height: auto;
    max-width: 40px;
    max-height: 40px;
    vertical-align: middle;
}