When a user uploads an image to my site, the image goes through this process;

• user uploads pic
• store pic metadata in db, giving the image a unique id
• async image processing (thumbnail creation, cropping, etc)
• all images are stored in the same uploads folder

So far the site is pretty small, and there are only ~200,000 images in the uploads directory. I realise I'm nowhere near the physical limit of files within a directory, but this approach clearly won't scale, so I was wondering if anyone had any advice on upload / storage strategies for handling large volumes of image uploads.