You want to change it for all your sites ? If it's the case, just replace the file named generic-site-32.png with your new image and restart the server.
Ok. In this case, I think that you have to modify the my-site.js (and my-site-min.js) file. In the renderCellIcon function, the link to the image is created. You have to do a switch, in function of the site Name.
case "site1" : img = site.isInfo ? "help-site-bw-32.png" : "filetypes/site1.png"; case "site2" : img = site.isInfo ? "help-site-bw-32.png" : "filetypes/site2.png";