As long as they are public domain or otherwise under a open content license (i.e. Creative Commons without NC clause). I don’t know about this source in particular but many are not actually careful about this — be specially wary of any site which claims everything is free or puts CC license tags on everything without actually saying the content creator.
And, of course, make sure to give any proper attribution in the article and in the image catalog metadata in WordPress.
The lack of any attribution and CC status was my concern. I’ll steer clear.
Thank you, @mattdm