I think everything going back to the Kindle Keyboard now has support for the new KF8 format. I don't know of problems resizing images, when I author Kindle books they seem to work out OK.
Here's my to do list for the Calibre plugin:
- Cope with KF8 format
- Better user interface
- Support "location offset" (like the .NET version)
- Support aliases list (like the .NET version)
- Support "spoilers" option (like the .NET version)
Any suggestions for what priority these should be in?