After you have clicked on 'Post Reply' and the box where you type your reply has opened scroll down the page to find 'Upload attachment' - click on it
The option to choose the attachment will appear - click on 'choose file' and the Windows "open" window will open allowing you to browse to the image you want to attach - select it - and then click the 'open' in the bottom corner of the 'open' window
this will take you back to the forum with the name of the file displayed - click on "Add File"
After a few seconds as the file uploads you will then be shown this screen - put the cursor in your text box where you want the image to appear and click "place inline" - this will put the attachment text in your reply box so
Finish typing your reply and press the 'submit button as normal