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The width and depth of an image are not all they seem, unfortunately. You need to know about resolution too. Common settings are 72, 96 or 120 dots per inch for monitors. Let’s use 100 dots per inch to make things easier in this example. If you have an image that is 2100 pixels or dots across and 1500 deep then it will appear on your screen as measuring 21” by 15”. In fact, unless you’ve got a really big screen only part of it will appear and you’ll find yourself scrolling around to see it.

Just to complicate things, some images you’ll get may be what are called high resolution images, maybe at 300dpi. The same image of 2100 x 1500 will in fact be a reasonable sounding 7” by 5” pic but when you open it, because your screen only shows at 100dpi, you still have the same huge image in front of you as you had before.

The reason for high resolution images is purely for printing purposes, and the higher the number of dots per inch, the more accurately the printed image will reflect its subject. Professional photographers and graphic designers demand higher resolution than you’ll need to worry about in most teaching activities (outside that curriculum, of course). All you need to do is choose between terms like quick, normal or best when printing and with most modern printers, normal will suffice for your day-to-day purposes and you shouldn’t need to worry too much.

So, think about whether the image is for screen or for print. For screen use, most image editors’ normal or 100% view will be roughly what users will see on their screens. Unless they all have huge monitors or you really do want to fill their screens to overflowing then dimensions rather less than 600 by 400 pixels will be what you need, and probably much less for smaller illustrations within text. For print, just remember than an A4 portrait sheet measures about 6” across and 10” down between the margins so you’ll be unlikely to need anything bigger.

Only rarely will you need to change resolution but it can be helpful to understand a bit more about why things don’t always look the size you expect on screen.

For the 1st picture the resolution was left at its original 72dpi and the dimensions changed to 6” by 4.5”. The screen shows it at about actual size.

For the 2nd the dimensions were set at 6” by 4.5” but at a resolution of 150dpi so more dots were needed and couldn’t all be displayed in the same size window. This would produce the same size print as above but at better quality

When the resolution was dropped to just 10 dots per inch, the 3rd screen display shows a very small picture, still usable on a web site perhaps although the printed size will still be the desired 6” by 4.5”

Finally, magnifying the image to about the same screen size as the others shows just how poor the print will be. Each individual pixel or square ‘dot’ being clearly visible.