Where can I get an explanation of the various graphic images that appear to the left of the thread title - flame(?), cyclist, '+' sign, etc.

A flame means the thread is popular, a plus appears on threads you've posted in have new posts, a arrow pointing to the left indicates a thread you've posted in, and a lock is... self explanatory.

Go to the index page for any given forum (Commuting, General, Road, any one will do- even this one) and scroll to the bottom of the page. You'll find the icon legends there. Note that the main index page for site has a different icon set as well.

Thank you both.