Ok, so you're using photoshop right. When you save the image, go to File>Save for web. Go to the settings pannel and make sure the box that sets the file type is either at PNG-8 or JPEG, depending on what file format you want. In the bottom pannel, there should be some information about the new file size. Already, it should be much lower than it was before, but you can further reduce the file size by playing around with the values in the settings pannel. Keep in mind that in general, the more the file size is reduced, the lower the quality of the image will be, so look for a good medium in between file size and quality until you're happy.