How can I find the name of the field store in the DB?
Drupal 7

If the name of the image field is "Dark Image", then the table name in database will be field_data_field_dark_image

Drupal 6

As mentioned in this page, table name should be content_type_YOUR_CONTENT_TYPE_NAME. If the name of the image field is "Dark Image", inside the table you will find a column called field_dark_image that stores field data.