Preheat the oven to 450 degrees F. Whisk the mayonnaise, lime juice, chile powder and 1 tablespoon water in a small bowl; cover and refrigerate. Put the tostadas on a baking sheet and bake until crisp, 5 minutes.
Heat a large skillet over medium-high heat; add the raisins and cook, shaking the pan, until plump, about 30 seconds. Add the olives, garlic and bay leaves; cook, stirring, 20 seconds. Add the pico de gallo, olive brine and 2/3 cup water. Simmer until thickened, about 8 minutes.
Add the tuna to the skillet in chunks and cook, spooning the sauce on top, until heated through, about 5 minutes. Sprinkle with the parsley and remove from the heat. Discard the garlic and bay leaves.
Spread about 1 tablespoon of the chile mayonnaise on each tostada. Top with the tuna and sauce, the lettuce and the remaining chile mayonnaise. Serve with lime wedges.