To get the block to show different icons for different roles, create an icon in the i directory of the theme being used and call it roleX.gif, where X is the roleid.

e.g. Where Teachers are role 3 and the theme flashywhizzy is being used, which has custom icons, the path would be moodle/theme/flashywhizzy/pix/i/role3.gif

Where custom icons aren't used, you'll need to add it to /moodle/pix/i