I have come across similar problems and somehow CDATA wrapping didn't work for me. I had to override the rss view and create the cdata tag and wrap image inside it programmatically.
I have done this by overriding the RSS view created by views_rss and views_data_export.
Here is how my view setup looks like:

To RSS via custom tpl display I created another display called as Feed which is a data export view of first display.
Here is how the feed display look like which I have overridden.

Next I overrode the Feed display tpl's and created the CDATA image field programmatically.
Here is the view tpl files that I overrode.

Now you would need to preprocess rss view in template to make node's nid available to the views.
Here is how I did that.
function bootstrap_preprocess_views_view_row_rss(&$vars) {
$view = &$vars['view'];
$options = &$vars['options'];
$item = &$vars['row'];
// Use the [id] of the returned results to determine the nid in [results]
$result = &$vars['view']->result;
$id = &$vars['id'];
$node = node_load( $result[$id-1]->nid );
$vars['title'] = check_plain($item->title);
$vars['link'] = check_url($item->link);
$vars['description'] = check_plain($item->description);
//$vars['description'] = check_plain($node->teaser);
$vars['node'] = $node;
$vars['item_elements'] = empty($item->elements) ? '' :
format_xml_elements($item->elements);
}
Lastly here is how I created the CDATA tag programmatically with my custom image style.
Code for two view templates:
1. Display template
<?php print "<?xml"; ?> version="1.0" encoding="utf-8" <?php print "?>"; ?>
<rss version="2.0" xml:base="<?php print $link; ?>"<?php print $namespaces;
?>>
<channel>
<title><?php print $title; ?></title>
<link><?php print $link; ?></link>
<description><?php print $description; ?></description>
<language><?php print $langcode; ?></language>
<?php print $channel_elements; ?>
<?php print $items; ?>
</channel>
</rss>
- Row template. Here I am creating CDATA and item tag programmatically.

Hope this helps.