Gravity is still the best Twitter client for Nokia / Symbian S60 phones. A recent update (it gets a ton of alpha / beta updates all the time, and they're super simple to upgrade to directly from within the app) added direct posting to Flickr. There is a spot within the app where it asks for the "Flickr code". I couldn't for the life of me figure out where to get this code.
The answer is (sort of) simple. The first time you attempt to post an image, a dialog pops up with a URL in the title. That URL is http://mobileways.de/flickr
It's easiest if you visit the URL with your desktop browser, which will then end up in the standard Flickr authorize dialog, and once you give permission, you'll get a 9 digit code to enter. Take this code and save it in the settings when you are looking at the images section of Gravity, and you'll now be able to post directly to your Flickr account.