If what you’re asking is “how to allow the viewer to view one of the still images used in the animation, but not animated”, there’s a couple of solutions.
For both, you need to import the still image file as an asset into Encore.
1. Make a menu that shows only the still image. If you want the menu to only last a certain amount of time, uncheck “Hold Forever”, input a duration and set the end action to go back to the feature menu (or wherever else you want it to go). You can add a button for them to jump to a menu manually, make it invisible if you want (edit the menu in Photoshop and turn off the visibility switch for all the layers in the layer set, but not the layer set itself). Or just rely on the user to press the Title Menu button on the remote, and make sure to set that value in the Project properties.
2. Put the still in a timeline. Slightly easier to set up, really. I probably should have put this first.