Go to the Guest Messages tab by clicking on My Updates from the main page.
Click in the box "Leave a message for..." and a larger text box will open. Just above the text box to the right you will see a Insert Photo icon. Click on that icon and navigate to where the photo is saved. Select the photo and click insert.
You can also drag and drop a photo into the text box.
Photos can be added before or after any text you type into the box. Just make sure your cursor is where you want the photo displayed.