There are 2 ways to get at the full-size images. You can use the mouse and left click on the thumbnail image, or choose Show Full Size from the IMAGE menu. Both are shown below:
Left-click on the thumbnail image:
The full-size image opens in a separate, scrollable window (so if the image is too large for your screen, the window itself will still fit, and you can scroll to view parts unseen.) Below is a typical pop-up window.
Clicking with the mouse anywhere on the popup image will close the window.