site/cache folder
Contains downsampled versions of the images. The files have the name structure WIDTH-HEIGHT-name.jpg. Even if the original file is PNG or GIF, the downsampled files are all in JPG.
The files are created on demand by inc/image.php on request by the images parser in inc/parsers/images.php.
The folder may have to many items so that you may not be able to inspect it with FTP.
The folder is not backed up, as the downsampling can be redone by the originals. There is no timeout however for the files.
It may be necessary for you to delete a specific downsampled image because it is outdated or corrupted. You can go to the Image page of the image and you will see a link Image Cache Refresh. This link deletes all cached downsampled version of this specific image.