I'm a big fan of Jeff Bullas and found this infographic (produced by Salesforce.com) on his blog. It gives a great overview of how to put together the perfect Facebook post using images.
The main points to consider are:
1) Add a call-to-action
2) Use a simple and correctly sized image (300x300 pixels)
3) Create for mobile users (almost 70% of fans will see the post on a mobile device)
4) Engage by asking questions both in the original post and in the comments section
5) Check your metrics
