For the avatars, you could probably use some CSS. I know @Shelley has done some post bit decors for other occasions like halloween and you could probably base your code around them and adjust it to suit.
For the logo, you can edit it within a photo editor like photoshop and add a santa hat.
There's code in the support area somewhere where i posted a santa hat using the messageuserinfo helper but unsure where that is which will do this though you could use exisiting code submitted in the rm and adjusting it to suit would help you also in accomplishing this.
Here's a quick effort though if you can get one of sheldons logos since the hat looks better from his screenies.
I was actually referring to Sheldon's post. Since it's not transparent and he provides quality services I'm assuming by him posting the imagery you can contact him. I wasn't saying your thread was a paid request to authors which was why I posted a transparent usable image.