Look for the envelope icon at the top of the page, next to your username. What used to be PMs are now called conversations...the benefit of the change is that each message used to be counted separately; now you can store full conversations with X number of people (you may have to delete some old PMs to make room).
To post attachments, scroll to the bottom of the reply box and click Upload File.
What more help do you need with it? Click on upload a file, as stated, then select your image. The most common issue is with sizing...which can be addressed in a variety of ways.