To do this, you need to have graphics software to edit your images. You can get a graphics editor or use an online tool like Photoshop Express Editor or JPEGmini.
A free-to-use online Photo Editing by Adobe® Photoshop®. Use this if you need a quick fix — crop, straighten, adjust color, and touch up imperfections.
A free-to-use online service for compressing JPEG files up to 80% without any perceivable loss. It works only on JPEG files and generates JPEG files again as the output.