You have to have a program built in to your computer that messes with photos. I have Irfanview loaded on my computer. Its a free photo program. Once you have it loaded, then you just click on "resize" and pick the size you want, then save it.
you might already have a photo editor. I know that my computer came with microsoft picture manager.
I use tinypic.com. It automatically resizes them and you don't need an account or anything. Its just a free website. Just upload any size photo, then copy/paste the IMG code into you forum post.