If you choose the Display Image or Display Cutout Image activation actions, or the View image internally option in one of the DataLink-related actions, TOPCAT will try to display an image in an internal image viewer.
On Debian, the default image viewer (SoG) is not available and the fallback viewer is used, which just displays the given image in a basic graphics window with no manipulation facilities is used. The fallback image viewer looks like this:
Fallback Image Viewer
Alternative options are available to communicate with external image viewers.