Professional Publications
Some professional, high-end printers may require images to be up to 600 ppi for printing. Always check with your printer/publisher about required image resolution before submitting images.
Non-Professional
Non-professional printers such as inkjet, laser, and other common printers can best print images that are at least 200 to 300 ppi and higher. For images that just need to "look good", 200 ppi will work. Photographic prints are recommended to be at least 300 ppi. Images for large format poster printing can be around 150-300ppi depending on how close the image will be viewed.*
Screen images are different than images for printing because we must think about the pixel dimensions of monitors, TVs, projectors, ordisplay, rather than PPI. Use PPIfor printed images, but use pixel dimensions of the image are what really determine the size of the image and quality of how it will display on the web or devices.
Web
The idea has been for many years that images should be saved with a resolution of 72 PPI. . But the common misconception is that this resolution or PPI value is the deciding factor of image quality for web images, where it is actually about pixel dimensions (http://medialoot.com/blog/high-resolution-web/).
Each monitor is different and has a different resolution, so it makes it difficult to design a website that contains images that will display perfectly on every type of display. Over the years, technology has improved and so has the quality of our displays.Most popular are Apple's new retina displays that are on the most recent Macbooks, iPhones, and iPads.
If you're a web developer, check out some ways people are designing for retina or high pixel density displays.
Projector / Powerpoint
Just like web images, images for projectors should match the pixel dimensions of the projector. As with computer monitors, projectors also have their own display dimensions. For example, most 4:3 aspect projectors have a display of 1024 x 768 pixels, so an image that is 1024 x768 pixels with a 72 PPIresolution would be an ideal image size to be displayed from a projector.
