It's these two lines you want:
document.write("<img src='<?php echo plugins_url();?>/drawblog/icons/eraser.png' onclick='drawblogEraserOn()' style='cursor:pointer' id='eraserbutton'>");
document.write("<img src='<?php echo plugins_url();?>/drawblog/icons/erase_all.png' onclick='drawblogClearAll()' style='cursor:pointer'>");