>Then I think the actual means ( something + CRC) to identify an image + its size is not the right way to do it

That is correct.

>Maybe it would be better to use ( something + its dimensions ), eg file1.jpg + height + width

Yes, but this implies that we control the file name that we receive. We wanted to see if there could be a possibility of working around this. Because, if someone sends us MyHouse.jpg and later on ThisImage.jpg, those will not be the same file name but could be the same image and we would have no way of knowing they are duplicate.