You can change your brigade mark/logo under Community tab > Brigade > Mark. You have to be brigade leader to be able change it.
Brigade mark dimensions are 24px x 12px. Can be
.png (recommended) or
.bmp format. File has to use color depth of 24 or 32 bit rate and the size must not exceed 1484 bytes.
Brigade Mark can easily be done with tools like Paint or Paint.NET.
See also: Brigade Name Change Card