On a windows computer, find the file in your folder directory, right click on the image and select Properties. Go to the Details tab and the information will be shown here.
On a mac, find the image file in your finder, right click on the image and select Get Info.
You will either need image editing software like photoshop or coral draw, or there are online editors you can use. This is a great example
Always reduce an image in size rather than enlarge it as it's quality will be lost.
Always crop an image to fit the dimensions rather than stretch it to fit as your image will be distorted.