Well, since you already know what het is, this will be easy. Double het means merely that the animal is heterozygous for two different traits. A good example is a double het for snow corn. The snake is het for anery and het for amel. Taken a step further, a triple het is heterozygous for three traits.
Since hets and double hets are visually indistinguishable from normals, a picture wouldn't be of any use in explaining it.