How to (ab)use image effects to serve API generated images?
If you are getting the image from another server/site you don't need an image field. If the link isn't going to change, you can store it in a text field and build a link like this

your_module_node_view($node, $view_mode, $langcode) {
    $url = $node-&gt;field_cool_image[LANGUAGE_NONE][0]['value'];
    //redefine field_cool_image to display your image
    $node['content']['field_cool_image'][0] = array(
        '#markup' = '&lt;img src="' . $url . '" alt="my cool image" /&gt;'
    );

}


otherwise build the link when the node view is built.

your_module_node_view($node, $view_mode, $langcode){
    //whatever method you use to build a url
    $url = my_build_url($node);
    $node-&gt;['content']['my_cool_img'][0] = array(
        '#markup' = '&lt;img src="' . $url . '" alt="my cool image" /&gt;'
    );
}