The new Sidekick Slide and Sidekick LX will be capable of sending MMS (images, audio, video, etc.) messages and doing so will be included with your plan, according to Joshua Karp over on BGR.
These new models will be the first Sidekicks capable of sending MMS messages and any new customers will automatically get put on the new plan, but existing customers upgrading to the new phone will need to call in to get switched over to the new plan.