Old imagery files I should imagine - WeatherCat never deletes imagery as it considers it part of the weather data (not when you turn imagery off, or change the source of the imagery). This is great when I'm scrolling through some of the storms from last autumn - not only do I get the numerical data/graphs but I can see the imagery as well. I have imagery channel 3 setup to drag the webcam image off our web-pages, so we get to see what it actually looked like out of the window as well as a time lapse as the data is scrolled back and forth.
Anyway, if you want to delete the imagery already there, check with the manual (page 147 in the current manual) for the location. General info about the imagery on page 37.
Cheers,
Stu.