The Image Asset from Entry plugin for Melody and Movable Type will look in the Entry Body for a non-local image URL. If found, that image URL is used to create a new image asset in Melody. An asset-entry association is created (to add the asset to the Entry Asset Manager). The resulting asset URL can replace the original, or the original URL can simply be removed. Lastly, the new asset can be associated with an Image Custom Field.
I wrote this plugin specifically to help me publish my photos on Flickr to my blog: when an entry is published through the XML-RPC API to the blog, normally the Flickr image URL is displayed in the entry.
If a Flickr URL is supplied, this plugin will also update the URL to grab the "Large" size image (1024 pixels wide). Using the Large image makes it easy to get exactly the size image you need because it's a big-ish file that can be sized with the AssetThumbnailURL tag to any dimensions required.
This plugin works for entries created through the API, and also works when on the Edit Entry screen: save an entry and the Entry Body is reviewed for any non-local image URLs that can be converted into image assets.
Take note that this plugin makes it easy to accidentally steal content: any image linked on another site will be saved to your site. For my use -- saving my photos to my blog -- this isn't a concern, however it's something to be aware of.
Melody 1.0 or greater
The latest version of the plugin can be downloaded from the its Github repo. Packaged downloads are also available if you prefer.
Installation follows the standard plugin installation procedures.
Publish an entry through the API or save an entry through Melody's administrative interface.
This plugin is licensed under the same terms as Perl itself.