Transparency in images suddenly is black

After activating Nitropack I noticed that the transparent areas of the user avatar images that are displayed within my Google reviews box (plugin “Reviews and Rating – Google Business”) have a black background. Before the transparent background was white.

Here comes a link to a Google support website where a user reports a similar issue:
https://support.google.com/google-ads/thread/28379913?hl=en

How can I make sure that the transparent background of the avatar images is properly displayed as white and NOT black?