There are only two image resolutions you need to know about.

The resolution for images used on websites, blogs and email is 72 pixels per inch. The resolution for images that are going to be printed with ink on paper is 300 pixels per inch. If you never print your images, you can go ahead and make all your pictures 72 pixels per inch. If you want to display them on a screen AND print them, make two sets of images – one at 300 pixels per inch and one at 72 pixels per inch.

In Photoshop, you can open an image and change the resolution using Image/Image size. Determine the resolution by how you intend to use the image.

Powerpoint = 72 pixels per inch
Word documents = 72 pixels per inch
Website images = 72 pixels per inch

Important: Always set the resolution before you change the size.

Images seen on the web have a lower resolution because that is the resolution of screens. There is no point using 300-pixel images on a website. They should be made for 72 pixels per inch and probably no wider or taller than 900 pixels.