Put the picture you want to post in a folder on your hardrive where you can find it. When you replying to a message, below the window where you are typing, click the box that says "embed picture in post." To the left of that hit "click here to upload." This will open another window- click on one of the browse buttons...locate the folder and then the photo...double click on the photo and it will upload. Presto, the picture will appear in your post.
BTW- Welcome to the T & TH forum!
"The joy of living is his, who has the heart to demand it." Teddy Roosevelt