Really depends on what you store the digital images on - that's the thing to degrade, not the image itself, that's just an abstract concept. Sadly, a lot of storage methods are sort of "all or nothing" - it works, until it doesn't and then you simply lost the images altogether. Degradation in the form of the image still being a working file, but "looking worse" is quite unlikely. In any case, if you want to keep your digital files (not only images) for a long time, there is no "store and forget" way to do it - to put it simply, digital data does not exist long enough for us to have a reliable long-term storage, because not enough time has yet passed to test it. CDs and DVDs last years, possible decades, but they all can suffer from degradation of plastic materials with time. The most reliable storage is probably a magnetic HDD (or even better an array of those), but one that is connected to a computer and accessed regularly, so that if it starts to fail, you can notice that and copy the data - an external HDD, unplugged and in the drawer is pretty much "Schrodinger's data" - it may be there or not. Any Flash/SSD is much worse as unplugged storage, as the charge WILL be gone at some moment in the future - nothing has infinite resistivity. Overall, the best way to keep your data is to keep it with two different methods and copy it to a new device once in a while.
