how to make a field in a view be the link destination to another field?
Make sure the Row Style is Fields. Add both fields, link field first and then image field. 

Edit the link field (in the Views UI) and check exclude from display and use the URL as plain text formatter. 

Finally, edit the image field and check output this field as a link. There are replacement patterns you can use for the link destination, the link field should be available there. You will need to check use absolute path.