There are several ways in which you can go about comparing the appearance of two or more images.
The most obvious is that you can create new image displays (see the "New Window" item in the "File" menu) and use the window manager to resize the displays to fit side-by-side.
The second way is to "blink compare" your images (i.e. put them in an aligned stack and rapidly cycle through them, something like flicking through a stack of cards). Before you can blink compare you need to display all the images in their own main windows. Now select the "Blink images..." item in the "Image-Analysis" menu. This toolbox has its own help description:
The third method for comparing images is by contouring. The contouring toolbox allows you to overlay contours from one image onto another (these can be aligned through celestial coordinates, so it's possible to overlay images with different pixel scales and alignments, e.g. radio maps over optical images). The contouring toolbox is available as the "Contouring..." item in the "Image-Analysis" menu and has its own help description: