In some photo software when you save the jpg you can select the quality, some even tell you how many kb the final file will be. You may need to resize so the photo won't blow the margins on the thread, but sometimes reducing the quality can help make it small enough to upload.
If none of the software on your computer will do that, and you don't want to download anything there are some web based services.
http://www.webresizer.com/ is an example google coughed up.