Open the Resource Manager You will see that there are three tabs at the top of the window. Manage Wordlists, Construct Wordlists and Manage Resources.
The main tab is Manage Wordlists. You will see that there is space for two lists A and B. To open the list you want to modify click on Open under the heading Wordlist A. You can open a second list in Wordlist B as a resource list from which you can select individual pictures to add to list A.