If you have lost your installation media for Photoshop, but you do have a legitimate serial number for Photoshop, you can install the trial version and enter your serial number during the installation process to convert it from a trial to an non-expiring full version.
Do be careful when seeking a free Photoshop download online. Many sites will try to trick you into thinking you are downloading a legitimate or official product, but the files they are distributing are actually malware or virus infected.
The safest place to obtain a legitimate download of Adobe products is from Adobe.com. Unfortunately, only the most recent versions are distributed as trials, though you may have some luck finding older versions of Photoshop on Adobe's FTP site. Start by drilling down into either the win or mac folder on the FTP server, and from there you will find folders for all the versions of Photoshop. There is no guarantee you will find an installer for the version you need, however. If you need to re-download an older version of Adobe Photoshop from a non-Adobe site, be very careful--do a search on the safety of the source and run a virus scan on anything you download.
If the cost of Photoshop is too much for you, consider Photoshop Elements instead. Photoshop Elements offers about 99% of the functionality of Photoshop at about a sixth of the price, and was designed to meet the needs of most non-professionals who will never use many of Photoshop's advanced features. A free 30-day trial version of Photoshop Elements is also available for download from Adobe.
• Comparison of Photoshop and Photoshop Elements
In March of 2008, Adobe announced Photoshop Express (now called Photoshop.com), a free online service for storing, sorting, editing, and showing off digital photos. Although it is not as rich in features as the desktop software Photoshop and Photoshop Elements, it is a legitimately free way to use Adobe Photoshop technology.
• Take a Tour of Photoshop Express
In addition, there are many very good photo editing programs that are free and offer many of the same features of Photoshop. To learn more, browse the free graphics software section of this site, or visit the following links for my top picks.
• Top Free Photo Editors for Windows
• Top Free Photo Editors for Macintosh