The year 2019 has not been kind to the Open Content community, particularly for those wanting to use CC0-/public domain-licensed images for remixing in presentations, websites, etc.
Many of the major platforms that have grown a reputation of making open, CC0-licensed images available for everyone decided to take a step back and introduce tailored licenses that would explicitly forbit the large-scale copying of content from their respective servers (a strategy that has led to the establishmend of "copycat platforms" such as Pexels, which mainly served content that had been copied from e.g. Pixabay and Gratisography, and which now has also introduced it's own license).
Now you might ask what's the fuzz about this?
All in all, it's about clarity - with universal CC0 attribution, people knew what they were using, and image creators had a way of making sure that re-use was allowed no matter what - making an image available via CC0 meant that the author/copyright holder agrees with others using this work in any way, be it commercially or not, and allowing alterations without the need of attribution (altough many of course appreciate it ;) ).
With the rise of platform-specific licensing, this universal intention has now been subverted - see e.g. irights.info for a detailed discussion of Pixabay's choice of a new license (in German)). For programmers, and with them theme developers in particular, this has further implications, as pixelrockstar notes, quoting Carolina Nymark: "Long story short, under the new Pixabay License the photos will not be GPL compatible".
But fear not, there are still a variety of options that can help you find image material that is licensed under Creative Commons Zero, thus allowing re-use for any open content whatsoever.
still the usual, including...