If you posted your artwork here in MT then all you have to do is go to your image, right click on it and select Copy Image Location. This will copy the URL where your image is stored. Then you come here, click on the picture icon in the Quick Reply box and paste the URL into the text field. Click OK and it should appear. If not, reply again, and I'll take a look at what you did wrong.
FYI, the URL you paste should end in a .jpg, .gif, or other image format that you uploaded your image as.