How do I show picture swatches as add-on choices?

Use a choice-type option and add an image to each choice:

1. In Products → Add-ons, select the group and click + Add option.
2. Set Type to select, radio, checkbox or color.
3. In Choices, put one choice per line. To add a swatch image, write the label, a pipe (|) and the image address, for example: Black | (image address). A line with only a label shows as text.
4. Set a Price and Price type if picking this add-on costs extra (one price applies to the whole option, not to each choice), then click Add option. Avoid commas in choice labels, because a comma starts a new choice.

Check the product page to see how the swatches look.