[balinor]

I'm pretty serious, but if you really want to do something like that, you would use CSS to style the class that controls the header, and you would probably need to add a <div> tag with a class around the welcome text in customer/main/welcome.tpl.

Seriously though, animated gifs are a huge no-no in this day and age, they give a VERY negative connotation. Same with animated snowflakes, reindeer following your cursor, etc. Professionally designed Xmas banners are great, or adding a santa hat to your logo for example. Keep the animations and cheap graphics to personal sites and blogs - not an e-commerce site.