The Long Now Blog has some excellent advice on how to increase the odds that those pictures and documents you’re regularly backing up (you are regularly backing up your data, right?) will still be readable decades from now,
Format. Always try to use the most common and least proprietary file formats when saving your files. If you have important email files that you would like to preserve, save them outside of your email platform as simple text files. For other documents, it has been recommended that they be saved in the PDF or PDF/Archive format. Though the PDF format is technically considered to be proprietary, the fact that the source code is available and the PDF format is now universally accepted, it is a good solution for keeping your files and their context intact. It is important to remember that for image and sound files, the larger, higher quality files are much more readable and can be used in many more ways than files that have been reduced or compressed. Compressing files for storage or emailing causes significant data loss, so avoid this when possible.