Add custom URL to Image on a node
If I understand well your question, an alternative can be edit your image field ("Configure field" options) and select Rewrite results &gt; Rewrite the output of this field. Here you can make lots of things with your image, for example add a link from other previous view field, over your image.

here one example to string included in the rewrite field: 

&lt;a href="[field_link_test]"&gt;[field_image_test]&lt;/a&gt;


in this example [field_test_link] is a text field with the respective web address, and [field_image_test] is the field that comes with the image.



this image comes from Drupal 7, but this steps also are possible in Drupal 6