For the past couple months I have been working on font posters for myfonts.com.
There are two types of images that myfonts recommends to help display a typeface. One is a font flag, which originally was supposed to be 200 by 200 pixels but now, because of retina displays, should be 400 by 400 pixels. The other type is a font poster, a larger image that is twice as wide as tall. Originally they were supposed to be 720 by 360 pixels, but now the recommended size is 1440 by 720.
My toolbox for creating these images is rather limited, but I have been able to supply at least one for over 80% of my font families. One of my favorites is below, a display for an almost illegible set of typefaces called Flag Day.