The simplest method would likely be to either execute a command containing a "Run an Application" action, or do the equivalent from within the plugin (I.E. starting an external application), and displaying the image using an application that's already capable of doing so (E.G. a specialized image viewer, or a browser).
Third-party addons like those from HCS Voice Packs and VAICOM, and plugins from users, do display their own GUI directly, however that requires a good understanding of programming in .NET, and either WinForms or WPF.