One of the most annoying bugs with twitter.mailana.com has been the missing images that show up in the graphs. I'm only able to scan Twitter every few days, and when people change their profile pictures the old links no longer work.
The ephemeral nature of the portrait links is a known shortcoming of the API, and I was getting a headache thinking about the possible ways to work around it. I'd either have to scan more often and tax both my and Twitter's servers, or creating a dedicated service that would handle the housekeeping of making sure the URLs stayed valid.
Then I discovered Shannon Whitley's SPIURL project, which offered a free static URL for any user's portrait image. Just link to http://purl.org/net/spiurl/<username> (eg http://purl.org/net/spiurl/petewarden ) and you'll get the image data back for that user. I switched my graph viewer over to using those URLs, and now you should never see a missing image again. Thanks so much Shannon for implementing this!