The resolution of an image is very important in determining the size and quality of the print or screen view. Resolution is measured in dpi (dots per inch). You need 300dpi for a high quality print and 72dpi for good screen resolution (for website images).

FOR PRINT - Many times people don't understand that designers cannot use a photo that a client gives them from the internet with a 72dpi and use it in a printed project. The full size file will need to be sent for a high quality result.

FOR WEB - It is common for graphic designers that are new to designing for web to save their images with 300dpi thinking that 300dpi will make the image appear sharper. The only thing you accomplish with the high dpi is a slower loading time for the webpage. So always use 72dpi when designing for web. (more on how to improve loading time with saving images down later...)