Assume we have image field field_featured_image, but we want to have another image thumbnail field (for example this thumbnail needs to be wider):
Add another image field field_featured_image_thumb
Enable Views PHP module
Add the field_featured_image_thumb field to view, also Global PHP, enter below code into [Output code]

Always remember that Global PHP must be placed after two other fields and disable display for two other image fields.
Enjoy.