When you click on "Reply", it opens a page where you can write your text and attach pictures. Below the text box, click on "Attachments and other options". When that opens, you will see "Attach:" and a button to click to "choose file". Note that there is a size limit.
When your text is done and your picture finishes uploading, click on "Preview". You will see how your text looks, but (if I recall correctly) the picture won't load until you click on "Post".