If this behaviour has suddenly changed, then it might be caused by the settings being updated in Dimensions to store stock pictures in the Document Management System (ADM).
To check this, do the following in Dimensions:
Ribbon style menu:
From the menu select, System, Options, System Control Options
Check the Store Stock Pictures in the Document Management System option
If it is ticked, then, if you do not wish to keep pictures in the Document Management system, then untick this option and click Save.
Classic/Module Menus style menu:
Select System Control, Options, System Options from the navigation menu
Follow from step 2 above.