Depending on your level of expertise you should either:
1. Gather information and tools to build your first Gtk 'Hello World' application.
2. Get the source code for any open source Gtk-based image viewer to get view on basic approaches.
3. Start coding by reading the GTK manuals, sample code and available tutorials.
It is rather unlikely that anyone will post the final code for an entire Gtk application on a thread like this. Is this a homework project?