You have to add false to view.xml in YOUY_THEME/etc/view.xml to the "category_page_list"

<?xml version="1.0"?>
<view xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/view.xsd">
    <media>
        <images module="Magento_Catalog">
            <image id="category_page_list" type="small_image">
                <width>240</width>
                <height>300</height>
                <transparency>false</transparency>
            </image>
        </images>
    </media>
</view>

You can also check here http://magento.stackexchange.com/a/148911/43587

You have to add false to view.xml in YOUY_THEME/etc/view.xml to the "category_page_list"

<?xml version="1.0"?>
<view xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/view.xsd">
    <media>
        <images module="Magento_Catalog">
            <image id="category_page_list" type="small_image">
                <width>240</width>
                <height>300</height>
                <transparency>false</transparency>
            </image>
        </images>
    </media>
</view>

You can also check here https://magento.stackexchange.com/a/148911/43587