To resize an image in Photoshop, open the image and then open the Image dropdown menu. Keep in mind that you'll almost always want to make the image smaller. Enlarging the image will distort it and lower its quality. This is why most digital cameras are preset to take larger images. IMPORTANT: Don't use the original image file to create the resized version. Always make a copy first and work with that file.
The menu shown below will open, with the Width and Height of the image in pixels. In a moment, you'll adjust those numbers to make the image smaller.
Before you continue, it's important to understand the Constrain Proportions checkbox. When this box is unchecked, you may change the width of the image without affecting the height of the image and vice versa. The problem with this, of course, is that the image will become distorted. Most of the time, you do want this box checked. When it is checked a pair of chains appear to the right of the Width and Height units as show in Steps 2 and 4.
We do want to maintain the proportion of the width and height of this picture, so we'll check the box, making the chains appear. Now, when we change the number of pixels in the Width or Height boxes, they will automatically adjust to maintain that proportion. In this case, we changed the Width to 450 and the number in the Height box changed automatically to 338.
When you click OK, you'll be taken back to your resized image. A star will appear next to the file name to indicate that the image has changed. This is a helpful reminder to save our work. You'll also notice below that Photoshop is set to show the image at 50% of its actual size. To zoom in and out on the image, use the Command - and Command + keyboard shortcuts.
Use the Command + keyboard to view the image at 100%. If you're happy with the size, move on to the next step. If not, resize again as needed.
To save the new image, go to the File menu and select Save for Web.
For photos, use the .jpeg or .png file format (.gif is best for web graphics). If you intend to display the image on the web, 72 dpi is the standard. Make sure those settings are in place before you click Save.