Stephane,
You are right that the images take space on the disk. That is why, we recommend to disable Visualizer after you create and debug a new test and it went into "production". The purpose of Test Visualizer is to let you understand what happens in the application, when you are creating a new test, or when you are trying to fix an error and need screenshots for this.
TestComplete projects include the Post image on error setting that is enabled by default. It commands TestComplete to post the screen image to the log when an error occurs. To view the setting, right-click your project in the Project Explorer and select "Edit | Properties" from the context menu. This will invoke the project editor and activate its Properties page. Select the Playback category on the left, and you will see the Post image on error property on the right, at the bottom.