[pkscout]

i found that this addon places artist image here --> XBMC\userdata\addon_data\script.artistslideshow\ArtistSlideshow\

so i wonder if next time i listen to the same artist,
will it use whats already in there

or

it will redownload everything from the net again??

By default AS will use the existing downloaded images next time. If it detects new images, it will download those, but not the existing ones. There is an option in the settings to set a maximum size for the image cache. If you have that enabled, then artist images will be deleted as you reach the limit you specified. In that case AS would again download all the images when needed.