Thanks Roman & Dimitry for your prompt answers.
The problem with your suggested solution is that the "Banner" field hides when the author selects the 'Small Banner Area' option from the "HeroAreaType" drop down list.
What I need is for the field to remain visible but not required, so if the author wishes he may or may not supply a value for it. On the other hand, if the author selects the 'Large Banner Area' option from the "HeroAreaType" drop down list, then he should not be allowed to Save the page unless the "Banner" field holds a value. A validation rule like: ((HeroAreaType==0) && (!String.IsNullOrWhiteSpace(Banner))) || (HeroAreaType!=0)
Is this possible to achieve in the Kentico admin, or in any other way?
P.S. In order to solve my issue using your suggested solution, I would have to introduce a second non required image field e.g. "SmallBanner" with the opposite visibility rules. So when the author selects the 'Small Banner Area' option, then this field will become visible but not required.