.png!? Wow, uh... that's going to
kill a 56k dial-up user and makea very slow load time for most users nonetheless...
But, if you really want to continue... I suggest just using the "Find and Replace in Templates" feature vbulletin has.
Open the admincp and go to styles & templates > search in templates.
under the "Find and Replace in Templates" search for:
now, in the "replace with text" put:
BE VERY CAREFUL WITH THIS! THIS WILL REPLACE ANY IMAGE WITH THE .GIF EXTENSION TO A .PNG EXTENSION. ALWAYS BACK-UP YOUR STYLES!
Good Luck...
Jonathan