img.ly is my preferred image service. It supports having a caption/description field for uploaded images. When I use the img.ly website to upload a picture, this caption is attached to the picture and can optionally be posted as a tweet.

Would it be possible to fill this caption field from within Tweetie? Ideally, I'd like to be able to specify text for the caption that is not the tweet. Although, filling it with the tweet or a link to the tweet would be better than the current empty behavior.

One possible way to implement this would be as follows:
1) drag in a picture file to the new tweet window
2) (optional) right-click on the added icon in the new tweet window. from the contextual menu that pops up, have an option to 'Set description'. enter in whatever text goes into the caption field on img.ly.
3) when a tweet is posted, if the description attribute of an image is blank, fill it with the text of the tweet, a link to the tweet, or something

It seems like this would be useful for other image services as well.