Image resolution is typically described in PPI, which refers to how many pixels are displayed per inch of an image.
Higher resolutions mean that there more pixels per inch (PPI), resulting in more pixel information and creating a high-quality, crisp image.
Images with lower resolutions have fewer pixels, and if those few pixels are too large (usually when an image is stretched), they can become visible like the image below.
When you change the resolution of an image, you are saying how many pixels you want to live in each inch of the image. For example, an image that has a resolution of 600 ppi will contain 600 pixels within each in of the image. 600 is a lot of pixels to live in just one inch, which is why 600ppi images will look very crisp and detailed. Now, compare that to an image with 72ppi, which has a lot fewer pixels per inch. As you've probably guessed, it won't look nearly as sharp as the 600ppi image.
Resolution rule of thumb: When scanning or photographing, always try and capture the image at the largest resolution/quality.
It's better to have more information than not enough! It's much easier for image editing applications, like Photoshop, to discard any unwanted image information (reducing the size of an image) than it is to create new pixel information (enlarge an image).
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 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.*
*Viewing Distance Factor
Screen images are different than images for printing because we must think about the pixel dimensions of monitors, TVs, projectors, or display, rather than PPI. Use PPI for 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.
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.
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 PPI resolution would be an ideal image size to be displayed from a projector.
Web images work differently than printed images. With printed images, we must pay close attention to resolution to ensure we get a high-quality print.
For web images, we must focus on the pixel dimensions. Look at the two images below - one is 300ppi and the other is 72ppi.
(Pixel Dimensions 300 x 200; Resolution 300ppi)
(Pixel Dimensions 300 x 200; Resolution 72ppi)
Both of these images display at the exact same size even though their resolution varies. This is because the pixel dimensions are what really matter when working with web images. Notice that the pixel dimensions of each image are the same and therefore both images will display at the same size even though they have differing resolutions.