A picnicker is a person who is having a picnic: sitting outside with food they brought, often on a cloth in a park, on a hill, or by the water. The word names the person, not the food and not the day out. You picnic; you are a picnicker. It turns up more in writing and in news (“picnickers left bags on the grass”) than in easy talk. In speech people usually say “people having a picnic” or “we’re picnicking.” It is ordinary, a little British in feel, and not fancy. It is never a verb, and it is not the picnic itself.