Use MConverter to convert your images to WEBP. It's a newer format which uses more efficient compression.
This way, your photos will take up less space, while their quality remains nearly identical as before.
After you have downloaded your converted photos, you can delete the original image files from your phone or computer, and enjoy the extra free space.
Later, when you wish to view your old photos, you may convert them to PNG
without losing quality, or to JPG