Karma is typically represented as a wheel, where all things are circular and we eventually come back to the place that we thought we had left behind. The wheel is an evocation of the idea of karmic balance, and it is typically represented by a wagon wheel with an even number of spokes. In many cases, the spaces between the spokes are given a petal-like shape, making the wheel itself resemble a flower.
The wheel of fortune is a card in the Major Arcana of the tarot, and it stands for concepts as varied as luck, chance, destiny and consequence. It is a card that stands for taking responsibility for the actions of the past, a reminder of the effects of karma. The Rider-Waite tarot has the most popular depiction of the wheel of fortune, but any tarot deck will provide an image that you can use for a tattoo.
Ammit is a creature with a crocodile head and the hindquarters of a hippopotamus that devours the souls of the wicked in Egyptian mythology. According to the myth, when a person died, his heart was weighed against an ostrich feather. If the heart was too heavy, symbolizing too many dark deeds, the person was devoured by Ammit. This can neatly symbolize the fact that all actions, no matter how hidden, will be accounted for.
Hagalaz is a Norse rune that is depicted like a letter H but with the center crossbar running in a diagonal from right to left. This rune is associated with sudden loss and sudden change and also with the idea of karmic lessons. The rune itself usually stands for disasters and ordeal, but it can also indicate lessons that need to be learned before progress can be made.