To answer your question, you type your message in the message window as normal. If you scroll down a ways you will see a red button that reads MANAGE ATTACHMENTS. Click that and a small window will open. You will have to stretch it a little to see some CHOOSE FILE buttons. Click on the first one, navigate to your pic and then click UPLOAD. After you have finished uploading the photos you want to post, click CLOSE THIS WINDOW button at the bottom. You will find some more text in your Message window. You can hit the PREVIEW POST button at the bottom to see how things will look. If you like it, just hit the SUBMIT REPLY button.
After all of that, I just noticed there is a feature below the Message Window that says DRAG FILES HERE TO ATTACH! Did you try that?