I tend to use example code from multiple online tutorials when doing something new or unfamiliar. If you already have a news post thing working then you'd just need to integrate a file update tutorial/example.
I've been using two open source PHP content management systems recently, Website Baker and Joomla. I don't think either allows images to be uploaded via posts. obviously its a bigger job to customise one of those than to write a basic generic script/page.
If you'd like a hand to write a basic script for your project I'd be happy to help. I wrote my own code for a community wesite which allows visitors to post replies to articles (see http://www.tayport.org.uk/article.php?id=175) and a simple forum in another part of that site. I use captcha images to avoid automated spamming and a review process so that junk messages aren't posted. you can contact me via matthew.mcnicol at gmail.com