You need to set the interprete footage to widescreen in the project window of Encore then it will stay widescreen. If you don’t it jumps back to 4:3 inspite of the widescreen menu.
Put all your images in the timeline. Select the first one and apply the adjusments you want to make. Then right click on this clip and copy. Now select all the other images, rightclick and paste attributes. All images have the same adjustments.