The problem: multiple images would not show up even though they're named and placed correctly.

This affected my install of 1.3.8 install 3/25/08.
These contribs are installed:
editor__fckeditor_plugin_2-5
image-handler_2.0-zc137
ultimate_seo_urls_2-106
fast_and_easy_checkout
short_description
zen_lightbox_1-4

Around line 25 of additional_images.php:

$products_image_base = str_replace($products_image_extension . '$', '', $products_image);


replace with...

$extensionReg = '/' . $products_image_extension . '$/';
$products_image_base = preg_replace($extensionReg, '', $products_image);


str_replace doesn't accept regular expression args... preg_replace does.