Just like magnets become weaker over time, the magnetic properties of a hard disk will diminish in the very long term and can be affected by environmental factors such as strong magnetic fields. The materials which are used to make CDs and DVDs decay over time and the problem is that even minor changes in the data can make the whole disk unreadable.  Different brands and different grades of optical media advertise different life spans.  It also depends on how they are stored, how well you take care of them, how often they are read, etc. 

But regardless of the above lifetime issues, even a simple scratch can render your CD or DVD unreadable. Hard disks are high precision mechanical devices spinning at high speeds, typically 7,200 rpm.  So there is always the possibility of failure due to a shock or a power surge. But even without a physical failure, it is possible that you suddenly lose the content of your whole hard disk, e.g. due to corruption of the file structure. So it is important to have multiple backups because data recovery is tricky, as explained below.

Data Removal

Sometimes you may have the opposite problem: getting rid of your images permanently, e.g. destroy old backups or cleanup up your hard disk before you sell your computer. CDs or DVDs are easy to destroy, but securely erasing data from your hard disk is not as straightforward as it seems. There are plenty of affordable recovery programs which can recover data from a formatted hard disk. Formatting the hard disk, then copying dummy data to the hard disk until full capacity, followed by a format will prevent software based data recovery and should be sufficient for most of us (1).

(1) Advanced note: (expensive) hardware based recovery techniques used in forensics and intelligence agencies can reconstruct the overwritten data based on physical differences between areas which have been "zero" for a long time and areas which have been "one" for a long time and which were only recently changed into a "zero" via overwriting and erasing. More sophisticated erasing programs with multiple and random write-and-erase cycles will make even hardware based recovery impossible.

Long Term Storage: "Migrate, Consolidate, and Refresh"

If we think in terms of decades instead of years, certain media will become useless in terms of capacity, or incompatible, or both. A typical example is the floppy disk which can barely store a single 2 megapixel JPEG image and few computers still come with a floppy drive. 

In the nineties, I used 80 MB "magneto-optical" disks (2).  My magneto-optical drive only had drivers up to Windows 98, so I recently migrated these onto my hard disk via an older computer which still had a parallel port and Windows 98.

(2) They have characteristics of optical and magnetic storage and are very reliable. Currently they come in higher capacities but are less frequently used, rather expensive, and require a dedicated reader/writer.

Some of the CDs burned with older burners are no longer recognized by newer drives.

So to avoid compatibility issues, it is advisable to migrate your data to newer media. Since capacities of magnetic and optical storage are constantly increasing, you can at the same time consolidate your data. For instance, 500 floppies can be consolidated into a single CD, 58 magneto-optical 80 MB disks can be consolidated into a single DVD, 275 CDs can fit on a single 200 GB hard disk, etc.  So once or twice a decade you will have to migrate and consolidate your old and small capacity media to the new and larger magnetic or optical media that become available. This has the additional benefit of "refreshing" your data to overcome the earlier mentioned issue of long term data stability.