Adobe has made an old version of Photoshop CS2 available for free...that's right FREE! You can download a copy here. Note that the installation process requires a serial number. Adobe has provided one on the download page under the Publisher's Description section. This free version is available for Windows and Mac computers.
DPI and PPI are abbreviations used to describe resolution in digital photography. Resolution means different things in different situations. And there is no standard way to describe it. Most commonly, people refer to PPI and DPI as the same thing, though technically this is not accurate. Let me explain.