If you were to click Reply, click the 'Go Advanced' button. Under the box where you type your message, you'll see a 'Manage Attachments' button. Clicking that opens a new window, from where you can click 'Add Files' and upload. Once uploaded, drag to the bottom half of the window. Click the 'Insert Inline' button if you want it to show up in the post as a thumbnail, and not just as an attachment.