Pretty much just like you upload your profile photo, except when replying, you click on Upload a File (to the right of Post Reply), choose a photo and click on whatever suggests that you'll be uploading it (photo size limit is 1mb / 1024kb), then after you upload it, choose whether you want to insert full size or a thumbnail (to the spot where you've positioned your text cursor (the blinky line)) and then press Post Reply.