The way it works is upload images contain an encrypted code which will retire the image after a certain date. I can see why this was thought a good idea. Although I'm sure the reality is even if your uploaded images have an expiry date it'll still be the case that others upload embarassing photos of you which don't... and if you don't want your embarassing images in the public domain why would you upload them at all?
The problem I see is, assuming this is how it works, when I download an image might it expire at some point? As a bit of a hobby I collect images from movies I like. What if these images start expiring on my computer and I'm left with a load of dead files? Will I know which ones are immortal and which ones have an expiry?
Generally I think this is going to be annoying!