I have this in extra.less, taken from somewhere here, to set node icons to off.png (read) and on.png (unread) which are 73 x 41 pixels in size.
Code:
/* Forum read & unread icons */
.node--forum.node--read .node-icon i:before{content: url(off.png);}
.node--forum.node--unread .node-icon i:before{content: url(on.png);}
/* Subforum read & unread icons */
.subNodeLink:before{content: url(off.png) !important;}
.subNodeLink.subNodeLink--unread:before{content: url(on.png) !important;}
/* Adjust node icon width to match image */
.node-icon{width: 80px;}