If is a JPG file the image is already compressed but if the format is a TIFF or something, you need first compress it to a JPG format. You can do that using the GIMP a free open source software, like photoshop.

If your image is already a JPG image you only need to copress more the image, again using GIMP. Important: Check the balance between quality and size.

I hope that helps, see ya!

URL: Gimp.org