Image resolution is determined by the amount of pixels per inch (PPI) which make up the image. It's important to plan out the overall size of your final image and accurately reflect the number of pixels that will be needed to make the image crisp and legible. If you aren't working from a high resolution source file, you may notice that when you increase the size of your image but not the resolution; your image will look warped or blurry. The issue there is that you increased the number of inches but didn't match the number of pixels needed to properly fill in the image when the application tries to choose how to fill in the boxes that each pixel represents.

If you are scanning an image, be sure to capture a higher resolution image as your source file. It's much easier to scale your image down from a higher resolution than try to upscale a very low resolution image.

Professional/Non-Professional Publications
If your work is going to be printed professionally, the publishers may require files that are up to 600 ppi for printing. It is important to start from a high resolution source file and provide a scaled version with the adequate resolution in the requested common file type. 

Typically if you are printing on an inkjet, laser, and other common printer, an image of 200 to 300 ppi and higher will be all that is needed to produce a decent image. If you are printing larger format images for posters or presentations, you may be able to vary your resolution from 150dip to 300dpi depending on the amount of detail and distance from which the viewer will be looking at your image.

Web/Screen Resolution
Images that are meant to be digital only only need to consider the pixel dimension for the intended display and site resolution. These may still be outlined by the publisher but it should be noted that scaling an image works the same way on a screen as it does in print. The main difference is that you can often hyperlink the full resolution image but show a lower resolution thumbnail on a webpage. 

Images for Web
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.

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. 

When working with raster images (pixel-based) it is important to understand that scaling an image in programs, such as Word, Powerpoint, InDesign, or Dreamweaver, does not actually resize the image, but rather stretches images larger or scales them smaller. When scaling, the resolution is not adjusted to best suit the new size, rather the pixels are stretched and can appear pixelated.

The most common side effect of scaling an image larger than its original dimensions is that the image may appear to be very fuzzy or pixelated. Scaling images smaller than the original dimensions does not affect quality as much, but can have other side effects. For example, if you upload a very large image to a website and scale it down to a smaller size, the website still must load the full size version of that image and could cause the web page to load more slowly.

Exceptions:
A very small amount of scaling can be OK, specifically when scaling down. However, we strongly recommended resizing images in photo editors, such as Photoshop or GIMP, to achieve maximum photo quality.
 
Vector graphics (such as clipart and charts in Word or Powerpoint) are not comprised of pixels and therefore can be stretched to any size without loss of quality.