When you edit/create a post, there is orange text below the box that you type your text in that says "Attachments and other options"
Click on that text and a new section appears
One portion of that section says "Attach:", click the "Browse..." button in that section and a dialog will open to find a file. Locate the file you want to attach in your device's file system. It needs to be in GIF, JPG, or PNG format. Add the file with that dialog. If you have multiple files, you can add them with the "(more attachments)" function. Once complete, click the post button as you would any other posting.