When you say reduce the size, I presume you are talking about bytes and not dimensions. By the very nature of images, more detail requires more bytes. It also depends on what you're coming from. If you're reducing the size of a JPG, you should know that JPG uses lossy compression, meaning each time you open it up and save it again, you LOSE quality. If the dimensions are what you want, you're in a bind. You can reduce DPI as low as 72 (if it is going to be displayed on a computer screen - but not for printout). Other than that, save it as a JPG with the lowest quality setting that you think has an acceptable quality. That's about all you can do.