A system like this one will most likely inspire folks to add a lot of products.. With lots of products you will most likely get a lot of media in the form of images for each product.
Your system as it stands now creates 4 versions of each image in the form of thumbnails of various sizes. So right there we have 4x the number of images actually uploaded.
And all of them live in image/thumbnails .. this is a bad idea .. It wont take long before this directory will bloat to ridiculous size..
Modern ext3 file system will allow you to have virtually an infinite number of entries so you are unlikely to run into the natural limit of the filesystem, but the more files you have in there the slower your file access time will be thus possibly adding a noticeable drag on page loads. And you can't assume ext3 will be the file system. NFS has a limit of 32k..
I would suggest each category or even each product getting it's own directory.. Currently images appear to be {productid/type}-{imageid}-{size}.extension
Probably not a bad idea to have the product id be a directory.