Greencard application: I need to downsize a pic to 240kb, but it has to be 600 x 600 pixels, and I don't know how to get the size that small whilst retaining those pixel dimensions??? Help!

Hi! I'm trying to get the correct image size for a pic I need for a Greencard application for the US. My friend took the photo on a DLSR. I set up a Photoshop document measuring 600 x 600 pixels, the size needed by the application document, but I can't get the image smaller than around 550kb. When I "save as JPEG" in Photoshop, no matter how poor the quality slider, the image stays around 1MB. Is there another way to get the image down to 240kb @ 600 x 600 pixels??????