This is a question-request. Currently, the only accepted type is a standard image. However, Apple Icon Store is an image too and it proves to be the only type that’s portable enough to be compatible distributing scripts when a stock app’s graphics is required to embed in a dialog.
Various type of images are available. Just try !
set a to choose file of type {"public.image"}
display dialog "This is a simple test" with icon a
Various types are available, but that doesn’t mean that they will all display in macOS. The ancient WordPerfect Graphic has that file type (and your script shows it in the Finder) but it throws an error when I try to open it.
The topic is about the create image view command of DialogToolKit+.