How do we control who can access and use Design Tools?

As for the LTI tools, the Multi-Tool is hidden by default as an option that needs to be activated in each course navigation where it is to be used, so it's not likely that users who don't know what the tool is used for would activate it in their course. Though to be sure, when Design Tools is configured using the settings above, the left navigation button is stripped out by a little javascript even if the tool were made available in the left hand navigation of a course. 

Once installed, the Upload/Embed Image LTI tool is always available as a content editor LTI tool in the Canvas Rich Content Editor since it's a generally useful tool and doesn't require much training to effectively use.