How do I insert the URL of an uploaded image into a custom field?

You can also simply get the url of attached images.

http://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src

As @MDelcour already provided the function reference, you can use like this

[php]
$image_id = get_post_meta( $post->ID, ‘meta_key_name’, true );
$url = wp_get_attachment_url( $image_id ); // returns full URL
$image_html = wp_get_attachment_image( $image_id ); // returns image with HTML tag
[/php]

Sorry, I guess I should have elaborated.

I am using the WP-front-end-pro form to upload an image that is attached to a custom field called Thumbnail.. works great, except it inserts the image ID number. What I wold like it to do is insert the url for the image instead. Where can I change this in the plug-in code? Simply replace the image ID with the image URL when someone uploads the image.

That ID is needed when editing the attachment, otherwise it’ll break. One thing you could do, save the attachment URL in another custom field. Put the custom field name in the $field_name variable and another_field_to_save_the_url will have the attachment URL if any.

[php]
function wpufe_copy_att_url( $post_id ) {
$field_name = ‘my_custom_field_name_in_the_form’;
$attach_id = get_post_meta( $post_id, $field_name, true );

if ( $attach_id ) {
$meta_value = wp_get_attachment_url( $attach_id );
update_post_meta( $post_id, ‘another_field_to_save_the_url’, $meta_value );
}
}

add_action( ‘wpuf_add_post_after_insert’, ‘wpufe_copy_att_url’ );
add_action( ‘wpuf_edit_post_after_update’, ‘wpufe_copy_att_url’ );
[/php]

Worked Perfectly! Thank you!