Mailplane features an iPhoto plugin, to easily compose a new message directly from iPhoto. The current Plugin only works with iLife 06 (iPhoto 6). I'm currently working on an adapted Plugin for iLife 08. The next Mailplane update (due this month) will contain it. Meanwhile, disable the iPhoto plugin in Mailplane's "Photos" preferences and use "drag and drop" from iPhoto to Mailplane as a workaround.
Update (2007-08-23): Todays Mailplane release (version 1.50) contains an updated iPhoto plugin that works with iPhoto 5, 6, and 7 (iLife 08). As an improvement the iPhoto "Mail Photo" panel is shown. You can choose the desired size and then compose the message in Mailplane. Tip: Press Option key while clicking the "Compose" button to open Mail.app instead of Mailplane.