Using background images formatter for taxonomy in views I have an image field per taxonomy vocabulary that I want to set as background to each views-row. I have the view filtered by taxonomy term, and I have set the taxonomy_image field as background.

However, I don't know which CSS selector would be appropriate. The tokens available are c.date, c.page, c.user, random, site info, and users.
If you have not disabled Add default classes under Style Settings of image field then you can use views_field_FIELD_MACHINE_NAME class to play with css. 

For example If you have a field with machine name field_book_category_icon then after adding this field to your view you can get a class views-field-field-book-category-icon on front end when ever you render that view on your page.