42005 Mobile Browser Check Japan

OK, Simple one here: In Japan there are three major Mobile Phone companies. Docomo Vodafone AU Vodafone and AU can read PNG files from their mobile phone browsers but Docomo can only read GIF files. I am producing a site that will serve to any user and I need the site to serve up the file with , yep as you guessed, the GIFs for the Docomo users and the PNGs for the Vodafone and AU users. Guys, I'd like a quick simple solution but let me know what you think and Ill go throught the ideas. Simple for someone in the know I think. Cool. Cheers Chris