Magento – Magento Watermark have black background when transparent

I solved this problem by calling Varien_Image::keepTransparency(true) on the before it does the resize.

Unfortunately, there's no event we can observe here, so we have to extend/override Mage_ConfigurableSwatches_Helper_Productimg::_resizeSwatchImage.

Maybe someone else (you?) can propose a more future-proof solution.

etc/config.xml:

<?xml version="1.0"?>
<config>
    <modules>
        <YourNamespace_Transparentswatch>
            <version>0.1.0</version>
        </YourNamespace_Transparentswatch>
    </modules>
    <global>
        <helpers>
            <configurableswatches>
                <rewrite>
                    <productimg>YourNamespace_Transparentswatch_Helper_Productimg</productimg>
                </rewrite>
            </configurableswatches>
        </helpers>
    </global>
</config>

Helper/Productimg.php:

<?php
class YourNamespace_Transparentswatch_Helper_Productimg extends Mage_ConfigurableSwatches_Helper_Productimg
{

    /**
     * Performs the resize operation on the given swatch image file and returns a
     * relative path to the resulting image file
     *
     * @param string $filename
     * @param string $tag
     * @param int $width
     * @param int $height
     * @return string
     */
    protected function _resizeSwatchImage($filename, $tag, $width, $height)
    {
        // Form full path to where we want to cache resized version
        $destPathArr = array(
            self::SWATCH_CACHE_DIR,
            Mage::app()->getStore()->getId(),
            $width . 'x' . $height,
            $tag,
            trim($filename, '/'),
        );

        $destPath = implode('/', $destPathArr);

        // Check if cached image exists already
        if (!file_exists(Mage::getBaseDir(Mage_Core_Model_Store::URL_TYPE_MEDIA) . DS . $destPath)) {
            // Check for source image
            if ($tag == 'product') {
                $sourceFilePath = Mage::getSingleton('catalog/product_media_config')->getBaseMediaPath() . $filename;
            } else {
                $sourceFilePath = Mage::getBaseDir(Mage_Core_Model_Store::URL_TYPE_MEDIA)
                    . DS . self::SWATCH_FALLBACK_MEDIA_DIR . DS . $filename;
            }

            if (!file_exists($sourceFilePath)) {
                return false;
            }

            // Do resize and save
            $processor = new Varien_Image($sourceFilePath);

            // Begin additions by YourNamespace_Transparentswatch module.
            $processor->keepTransparency(true);
            // End additions by YourNamespace_Transparentswatch module.

            $processor->resize($width, $height);
            $processor->save(Mage::getBaseDir(Mage_Core_Model_Store::URL_TYPE_MEDIA) . DS . $destPath);
            Mage::helper('core/file_storage_database')->saveFile($destPath);
        }

        return $destPath;
    }


}

You must move your data-install-0.9.6.php to data/namespace_module_setup folder:

File Tree:

|----Namespace
|--------Module
|------------Block
|----------------Adminhtml
|--------------------Catalog
|------------------------Product
|----------------------------Tab.php
|------------controllers
|----------------Adminhtml
|--------------------IndexController.php
|------------data    
|----------------namespace_module_setup
|--------------------data-install-0.9.6.php
|------------etc
|----------------adminhtml.xml
|----------------config.xml
|------------Helper
|----------------Data.php
|------------Model
|----------------Resource
|--------------------TableOne
|------------------------Collection.php
|--------------------TableTwo
|------------------------Collection.php
|--------------------TableOne.php
|--------------------TableTwo.php
|------------sql
|----------------namespace_module_setup
|--------------------mysql4-install-0.9.6.php

More information here