Basic Image Viewer
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.
This image viewer is very basic, and looks like this:
Basic Image Viewer
Alternative options are available to communicate with external image viewers.
Note: earlier versions of TOPCAT offered a more capable image viewer called SoG under some circumstances. However, this only worked when the Java Advanced Imaging classes were present at runtime, and these are hard to obtain now, so this option has been withdrawn.