Cut a shallow X in bottom of each peach and tomato with a sharp paring knife and blanch in 2 batches in a 4-quart saucepan of boiling water 10 seconds. Transfer peaches and tomatoes with a slotted spoon to a bowl of ice and cold water and let stand until cool enough to handle. Peel peaches and tomatoes, then halve peaches lengthwise and pit. Cut peaches into 1-inch pieces. Coarsely chop tomatoes.
Cook onion, ginger, and salt in oil in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until softened, 3 to 4 minutes. Add curry powder and cook, stirring constantly, 1 minute. Add peaches and tomatoes with any juices, sugar, and lime juice and simmer, uncovered, stirring occasionally, until mixture is thick and peaches are tender but still hold their shape, about 8 minutes. Transfer to a bowl and cool, uncovered, then chill, covered, at least 8 hours.