Cache folder of Image Show GK4 module too big

Hello.

I would like to know when I delete a tab on the Image Show GK4 if the thumbnail are deleted or it stores at the cache folder of the module.
I see not my cache folder is 900 Mb and I have a warning about the size .
I m using 3 times the Image Show GK4 so you understand that I m having a problem.

Joomla 1.7.3
Image Show GK4ver. 1.16
Thanks

Please backup this directory and then delete all files, the Image Show module should rebuild only thumbnails for this published slides.

I have already delete them but the thing here is why the Image show module not delete them ?
I have 3 copies of the module in the site and I m changing daily the images.
In some weeks they will be again 900 mb .
I cant delete them all the time manual

Thanks