Halve and pit peaches, then cut into 1/2-inch-thick wedges. Transfer to a large bowl. Add lemon juice; toss. Add sugar and rosemary; toss. Cover; let stand, tossing every hour, for 4 hours (sugar should be completely dissolved).
Put peach mixture into a large, wide pot. Bring to a boil over medium-high heat. Cook until liquid is syrupy, 12 to 13 minutes. Crush lightly with a potato masher, leaving one-quarter of the wedges intact. Skim foam from surface. Discard rosemary sprigs.
Seal and process: Fill hot, sterilized 4-ounce jars with hot jam, leaving 1/4 inch space in each jar's neck. Wipe rims with a clean, damp cloth; cover tightly with sterilized lids and screw rings. Transfer jars, using tongs or a jar clamp, to the rack of a large canning pot filled with hot water, keeping jars upright; cover with water by 2 inches. (Jars should be spaced 1 inch apart and should not touch sides of pot.) Cover; bring to a boil. Process jars in gently boiling water 5 minutes. Transfer to a rack; let cool 24 hours. If lids pop back when pressed, they are not sealed; refrigerate immediately, and use within 1 month. Sealed jars can be stored in a cool, dark place up to 1 year.