Omnia Theme Switch has come quite a long ways. I should have a URL to download by Dec 1 2009, so please check back. There's quite a lot to offer, but in fair warning there are still some bugs and difficulties, but it works great for me and I'm thinking it's ready for a limited beta-test release....
Omnia Theme Switch - Application on the Omnia to switch themes. Has a preview screen now, too.
Omnia Theme Creator - Small Windows-based Visual c# 2009 app to cut existing graphics to proper size for portrait / landscape format for Omnia (240x400)
Omnia Theme Studio - Windows-based wrapper program for OTC which allows simultaneous viewing of portrait and landscape images and Start Menu colors on images.
Some current drawbacks:
-- Uses a non-standard file format for files: They're still .jpgs or .gifs or .pngs, but the programs look for .opp (omnia portrait file) and .opl (omnia landscape file) and .osc (Omnia Color file). Not hard to make them by hand if you really wanted to, and very easy with the OT Creator and Studio (2-5 minutes from jpg on your PC to Portrait and Landscape theme on your Omnia).
-- Isn't currently compatible with standard .tsk files.
Some nice touches:
-- OTS not only creates the themes, but if you have Omnia Theme Switch installed on the Omnia, it can send it to your Omnia and display it there.
-- Color file is standard text and can be edited at any time to alter color scheme.
-- I've got some themes I've already created and can be used as starting inspiration.
-- Fix the bugs! (I'm a developer, but this graphics stuff is Strange New World. Although I'm happy to have gotten this far!)
-- Build a self-extracting version of the program - compiles to cab files, to install just double-click or start the cab files. (I'm still learning about .cab files)
-- Adjust so program output is compatible with standard Theme software and the Today screen loader.
Let me know if you're interested so when I get it available I can let you know.