Interesting problem. Looks like there's two angles on doing this. One is to email the picture from your phone to your webserver and have it "picked up" by a PHP mail handler (looks like that's the idea with pivotlog).
XML-RPC-wise, seems there's KABLOG which is a Java app for your phone, able to talk XML-RPC.
This isn't a PHP solution, but if might be a time saver. My friend Kevin here in japan has created a service called MFOP2 (mobblogging for other people 2) that some may find useful for getting mobblogging started up quickly. http://new.bastish.net/cgi-bin/mfop2/index.cgi