Not sure if this will help you or not, but my files folder has a folder called imagecache and inside are cart, category, product, etc which I guess were created automatically. My file system does have temporary directory as /tmp, but I don't think this is doing much with the image cache. The image cache is likely what's causing you problems.

Check if you have imagecache subfolder under the files folder and then delete and reupload an image to test.