Image hosting sites

Hey all I'm looking for suggestions for free image hosting sites but I have a specific requirement.

Most hosting sites will be able to give you a direct link when you upload an image but I want to upload an image to overwrite the previous image but have it keep the same URL.

For example I upload a picture of a flower and give me the url
www imagehostingsite com/sw3221.png
I want to then replace that with a picture of a tree and then any where I posted links to that flower will now go to the picture of the tree instead.

Does any such hosting site exist?

you can do that with dropbox