Examine canning jars for cracks or chips and discard any damaged jars. Wash jars, lids, and rings in hot soapy water, then keep jars hot by running them through the dishwasher or submerging in hot water until ready to fill.
Fill water bath canner halfway with water and begin heating. Fill large pot with water for blanching peaches and bring to a rolling boil. Prepare large bowl with ice water.
Prepare light syrup by combining sugar and water in a large saucepan. Bring to a boil, stirring until sugar dissolves completely. Keep hot but do not continue boiling.
Wash peaches thoroughly under cold running water. Score a small X on the bottom of each peach with a sharp knife. Working in small batches, blanch peaches in boiling water for 30-60 seconds, then immediately plunge into ice water bath.
Remove skins from blanched peaches by pulling them off with your hands or a paring knife. Cut peaches in half, remove pits, and slice if desired. Treat with lemon juice if using to prevent browning.
Pack prepared peaches into hot jars using the hot pack method: layer peaches in a large pot, add hot syrup to barely cover, and heat for 5 minutes before packing into jars. Leave ½ inch headspace in each jar.
Pour additional hot syrup over peaches in jars, maintaining ½ inch headspace. Remove air bubbles by sliding a clean plastic knife or bubble removal tool around the inside of each jar.
Wipe jar rims clean with a damp cloth to remove any food residue or syrup. Center new lids on jars and apply rings until fingertip tight (do not overtighten).
Process filled jars in boiling water bath canner, ensuring water covers jar tops by 1-2 inches. Process pint jars for 20 minutes, quart jars for 25 minutes. Adjust processing time for altitude: add 5 minutes for 1,001-6,000 feet elevation, add 10 minutes above 6,000 feet.
Remove jars using jar lifter and place on towel-lined counter, leaving 1 inch space between jars. Cool completely for 12-24 hours. Check seals by pressing center of lids - properly sealed lids will not flex. Store sealed jars in cool, dark place for up to 2 years. Refrigerate any unsealed jars and use within 1 week.
