For in-universe articles, images should originate from an in-universe source, such as a screenshot from an episode. Promotional images - where an actor poses as their character for a photograph - are not allowed. For example, a promotional image of Connie Beauchamp is not an image of Connie Beauchamp - it is an image of Amanda Mealing pretending to be Connie Beauchamp. These sorts of images may be used on real world articles about the actor, though images of them out of costume are much preferred.
Out-of-universe and episode articles
For episode articles, promotional images also should not be used. Instead, screenshots that depict key moments from the episode should be used.
Promotional images mainly belong on articles about an actor or members of the Casualty production crew. Even then, an image of an actor out of costume is preferable to one that shows them in costume.
Much like how promotional images cannot be used on in-universe articles, screenshots cannot be used on real world articles. If you don't have a headshot or other promotional image to use on an actor's article, don't use any image at all.