How to Downsize Photos
To downsize photos in Photoshop, click "File" in the menu bar, then click "Open" and select your image file. Next, click on "Image" in the menu bar, then click "Image Size." Select "Bicubic Sharper" in the drop-down menu and enter the new dimensions by changing the number in the "Width" or the "Height" box. When you change one of the numbers, the other will adjust itself automatically to maintain the right proportions for the image.
Right-click on an image and click on Open with…. It's in the pop-up menu. Click on Paint. Paint automatically ships with most versions of Windows. Click on Home. It's a tab near the top of the window. Click on Resize. This will open the "Resize and Skew" dialog box. Check Maintain aspect ratio.  Set a new size for the image. To do so: Click on Percentage to enter a percentage in either the "Vertical" or "Horizontal" field and reduce the image size by a certain proportion. Click on Pixels to enter specific dimensions in the "Vertical" or "Horizontal" field. Click on OK.  Click on File in the menu bar, then click on Save as….  Type a name for the image in the "File name: " field. Click on Save. The image will be saved with the dimensions you specified. Open a photo in the Preview app. Do so by double-clicking on the blue Preview icon that looks like overlapping snapshots, then clicking on File in the menu bar and Open... in the drop-down menu. Select a file in the dialog box and click on Open. Preview is Apple's native image viewer app that's automatically included with most versions of Mac OS. Click on Tools. It's in the menu bar at the top of the screen. Click on Adjust Size….  Set a new size for the image. Select new dimensions or "Custom" from the "Fit into:" menu. If you select "Custom," enter the dimensions in the "Width:" and "Height:" fields, and/or the number of pixels per inch/cm in the "Resolution:" field. Click on OK.  Click on File in the menu bar, then click on Save. The image will be saved with the dimensions you specified. Open a Photoshop file. To do so, double-click on the blue app icon that contains the letters " Ps ," then click on File in the menu bar and Open... in the drop-down menu. Select a file in the dialog box and click on Open. Click on Image in the menu bar at the top of the screen.  Click on Image Size…. This will open a dialog box. Click on Bicubic Sharper (best for reduction) in the drop-down menu at the bottom of the dialog box.  Enter the new dimensions. Type them in either set of "Width:" and "Height:" fields (the other set will adjust automatically). You can also adjust the number of pixels per inch/cm in the "Resolution:" field in the "Document Size:" section. If you don't want to keep the same aspect ratio for the image, uncheck "Constrain Proportions" at the bottom of the dialog box. Click on OK.  Click on File in the menu bar, then click on Save as….  Type a name for the image in the "Save As: " field. Select an image format in the "Format:" drop-down menu. The default format is Photoshop. Click on Save. The image will be saved with the dimensions you specified. 
Right-click on an image and click on Open with…. Click on Paint. Click on Home. Click on Resize. Check Maintain aspect ratio. Set a new size for the image. Click on OK. Click on File in the menu bar, then click on Save as…. Type a name for the image in the "File name: Click on Save. Open a photo in the Preview app. Click on Tools. Click on Adjust Size…. Set a new size for the image. Click on OK. Click on File in the menu bar, then click on Save. Open a Photoshop file. Click on Image in the menu bar at the top of the screen. Click on Image Size…. Click on Bicubic Sharper (best for reduction) in the drop-down menu at the bottom of the dialog box. Enter the new dimensions. Click on OK. Click on File in the menu bar, then click on Save as…. Type a name for the image in the "Save As: Select an image format in the "Format:" drop-down menu. Click on Save. 