The one thing missing from my posting by email setup was support for images. The Mailsave module has finally been updated for Drupal 6, and so I can now submit attachments with email posts. The one shortcoming is that files are simply added to posts as normal attachments, and so images aren't automatically displayed. I therefore have to manually insert images in the body of the post, but I actually prefer this since it's a simpler system and gives me more control.
I also needed a way of resizing images on my phone since they are too big. I found the Nokia Image Editor1 which seems to work fairly well, although it only allows resizing to specific resolutions.