When you create a post or message, you'll have a text edit toolbar at the top of the text box. In that section, you should see a button labeled "Add a photo".
Here's how to add an image:
1. Click on the "Add a photo" button.
2. If the photo you want to use is on your computer, click the "Browse..." button and select the image you want to upload. If the photo is from another website, click the "Paste a URL" tab and insert the image URL into the text field.
3. When you're finished, click "Insert".