>Think you can identity the image you receive with CRC ( I'd rather use MD5 or other method)

Yes, the original image could be used to detect if it is the same or not. It is just that the CRC32 stored in the database is after the resize.

What about MD5?

>And on an afterthought - do you need to store the resized image ? Can you resize it just for display purposes when you need it in a certain height and width ?

Yes, this is like that for various reason. 50 million files will take a much bigger disk space if not resized. Also, this is for performance access. If the resize is done in real time, it will not be a fast as otherwise. Also, this is batch display. So, when displaying a lot of images, if they are resized, this will be considerable.

>Don't know which business it is - but could you throw away the previous one if they send you another ?

But, for that, I'd need to know if this is a duplicate or not.