When buying an image file what is important is the number of pixels.
The ppi (or dpi) of a file is just a way to measure it. For example, let's say you have an image containing 3200x4800 pixels.
If you print that on 4x6 paper, you're printing at 800ppi. (3200 pixels / 4 inches = 800 ppi and 4800 pixels / 6 inches = 800 ppi).
If you print that on 8x12 paper, you're printing at 400 ppi.
If you print that photo on 16x24 paper, you're printing at 200 ppi.
It doesn't matter if the ppi setting in the photo file is 1200, 300, 96, or anything else. When you print you will use your printer's software to set the paper size and what size to print it. For example, with a Canon printer use the printer's Page Layout setting to set up the paper size and size of the printed image.
When determining which size image to buy, decide if you need to print it, and if so, what physical size does that printed image need to be.
If you don't need to print it and just want to use it on a computer display (e.g. for a web site or Powerpoint presentation), 500 to 1000 by 500 to 1000 pixels is fine.
If you want to print it, you can figure out the size it will print it using these guidelines:
- Home ink jet printers print art at good quality from 150ppi. So if you buy an image that is 1500 pixels by 1200 pixels, you can print it as a 10x8 inch image (25x20cm)
- Color newspaper ads need about 150ppi
- Glossy magazines need 266-300ppi
- A high quality book needs about 250-350ppi
We save our image files at 300ppi, but as you can see from above what you really need to focus on is the pixel size of the image.
If you really need to change the ppi of an image file for some reason, you can use the Image Size dialog box in Adobe Photoshop.