Turn text or link field in image
The best way to implement this is to implement a custom display formatter for the link field. You can either do it directly in a custom module by implementing hook_field_formatter_info.

Or without try out Custom Formatters module if you want to avoid custom code:


  The Custom Formatters module allows users to easily create custom Field Formatters without the need to write a custom module. Custom Formatters can then be exported as CTools Exportables, Features or Drupal API Field Formatters.