So, assuming you have Media Center on your PC, you can extend it over to your XBox using this Media Center Extender:
Store all your digital media on your Media Center PC — photos, music, videos, live and recorded TV — and bring it all to life throughout your home through the Xbox console. The Windows Media Center Extender for Xbox also supports DVD movie playback.
Then some resourceful chap called Jon made an XBox Flickr plugin:
UPDATE: See comments - Jon got in touch and corrected me (thanks Jon!): he says:
ummm, i hate to break it to you, but this is for Xbox Media Center. Not Xbox Microsoft Windows Media Center Extender crippled rip off that MS charges its users 80 dollars extra for :)
Its the Open Source, free, and completely superior media center. Please correct your post :)
Good on ya, fella.