WP-Flickr plugin: legal matter

Hello,
I use WP-Flickr plugin. I love it. It randomly pulls photos from flickr.com — based on your input, and display them into your post. I credit the photo source by writing in the my post “courtesy of Flickr”.

As no any API Key from Flickr is needed to get it working, flickr.com is not notified for my use of their photos, so I wonder, is that legal? Am I infringing copyright by publishing these photos on my post?

Any opinion is welcome. Thanks.