First show the image tool by selecting the → check box. This will show up the Image section on the right hand side of the Parley screen.
To add an image to a word make sure the word is highlighted. In the Image section on the right hand side press on the Folder icon and choose the picture you wish to associate. On selecting an image for a pair of translations, the bottom right corners of the cells of both the words would be marked with a blue flag. If you want this way, do the same with the other word of the couple. Hovering over this flag with the mouse cursor shows the name and path of the image file.