How to strip  tags from teasers?
I have had good luck using the Smart Trim module. (This module is now available for Drupal 7 and 8.)

Choose the Smart Trimmed field formatter and check the Strip Out HTML checkbox to remove the image tags (and other formatting that you probably don't want in a teaser text).

It also gives you some other handy formatting options (limit to number of words or characters, add "read more" links, etc) and makes the formatting options available for fields in Views, as well.