[snakes1100]

There is by default no image for categories, those are the forums themselves that have the "squary images" next to them, in refrence those are the "status icons" for new posts or no new posts.

Do you want to replace just the two icons themselves or have a different on/off icon for each forum?

If you want to replace just the two default on/off status icons, just name your custom images the same names and upload them via ftp to the correct directory and over write the old ones.