What is the best way to alter image style in view for a teaser
While I would generally just access this from the field output type from the View. I would recommend looking into some theme adjustments (and preprocessing Views). Since you are outputting content to the Teaser, you could add some additional verification from the template (https://www.drupal.org/node/53464).

Some helpful links would be:


template_preprocess_views_view_fields
theme_image_style