Measure any of the above boxes. All should be exactly as tall as wide. A slight difference means
your monitor is slightly out of adjustment. A significant difference usually means your resolution
is set to a non-standard aspect ratio. Standard aspect ratio is 4/3. Non-standard is commonly 5/4,
the most common of which is 1280x1024. Using a 5/4 ratio may mean everything displays shorter than
it should, which is usually unflattering to pictures of people.
320px X 225px
What a 320x240 image can
look like using 5/4 resolution on a standard 4/3 display
320px X 240px
4/3 Aspect Ratio
320px X 256px
5/4 Aspect Ratio
Using a 5/4 ratio on a standard PC display means everything may display shorter
than it should. Looking at the 320 wide rectangles, you might think that a 5/4 ratio would result
in everything displaying taller than it should. It doesn't, because your display doesn't have that
extra height, and the extra vertical resolution is compressed into the available physical space.