Height is optional, if no height is specified the image will be a square. Example:

Colors are represented as hex codes (#ffffff is white)
The background color is optional and defaults to gray (#cccccc)
The text color is optional and defaults to black (#000000)

Adding an image file format will render the image in the proper format
Image format is optional and the default is a png
jpg and jpeg are the same

by default, images are rectangles (or squares)
This is optional, but you can force the image to be a circle
height will be ignored when shape is a circle

Custom text can be set and size can be adjusted
This is optional, default is the image dimensions (300x250)
a-z (upper and lowercase), numbers, and most symbols will work just fine
&hidetext=true can be used to hide any text
&textsize=20 sets the font size to 20 pixels

The font used is from the freely available M+ Font Project