Windows 7/8/10


Navigate to the image file on your computer and right click "Open with" and then select "Paint."

With the image open in the paint program, go to the ribbon and select "Resize."

From here you can change the size of your image by percentage, or pixel size. 
Once you are happy with the changes you click "Ok" to apply the changes. 
If you are not happy with the size you can click the "undo" button on the top of the screen, alternatively you can press "CTRL+Z" to undo as well.

Once you are satisfied with the image size click "FILE>SAVE AS" to save your file to a location of your choice. 

(Be mindful of what file extension you use to save your image as. Common image formats are usually .JPG, .PNG, and .GIF)

MacOS


Navigate to the image file on your Mac, right click the image (if using a track-pad put both of your fingers on the pad, and then press down to activate the right-click menu option.) 

Hover over to the option "Open With" and then select "Preview."

Once the file is open in the "Preview" application hover to the top of the screen to the toolbar and select "Tools>Adjust Size"

A display box with the current image dimensions will appear, from here you can adjust the length and width of the image by pixels, percents, or other measurements.
If you make a mistake or do not like the size of the image after clicking "Ok" you can click "Edit" on the toolbar, and select Undo. Alternatively you can press ⌘(Command Key) and Z to undo a change.

Once you are satisfied with the size of your image, click "OK" then scroll up to the toolbar "File"

If you wish to save this image as a separate file select "Export..." and save it to a location with your desired file name. 
This means it will not make any changes to the original file but simply make a copy of it with the adjusted image size.

If you wish to save this file as is without making a copy, simply click "Save" or ⌘(Command Key + S) (This will overwrite the original file.)

(Be mindful of what file extension you use to save your image as. Common image formats are usually .JPG, .PNG, and .GIF)