I don't, as a general rule, use Flickr. Not that there's anything wrong, but I don't really need a new photo hosting service. All my good images - maps and high-quality images - usually find their way to Wikimedia Commons. My rocketry pictures and other personal stuff, I just upload directly with Picasa and Blogger, or I upload them on a forum like TRF if I use them there. Not all animated GIFs work directly in Blogger, so I have a spare Imgur account lying around somewhere.
That's not to say, though, that Flickr is bad. In fact, I'm quite fond of Flickr, because unlike most image hosts, they give users a setting to release images under various Creative Commons licences. Only a small number are so licensed - 196 million of 5 billion, or about 3.9% - but 46 million of those images have Attribution or Attribution-ShareAlike licences. Those types - which permit derivatives and don't restrict commercial use - qualify as free images.
That means that they can be used, without restriction, on all Wikipedia articles. 46 million images represent a body of images 4 times the size of Wikimedia Commons, the normal image source for Wikipedia. Since Wikipedia doesn't allow linking external images, there's a system set up where free images can be migrated to Commons.
I've started transferring a few images, mainly to fill the gaps in the current MBTA images. There are some very good photographers on Flickr who allow free use of their photos. Here's a sampling of them; click on them to go to the Commons file page for embiggening and attribution.
MBTA Commuter Rail at Providence:
Davis Square on the Red Line:
Streetcar at (pre-renovation) Mattapan:
Blue Line platform at Government Center:
RIP Chuck Berry
3 days ago