I was going to point you too a App called Quick Resizer on the Mac App Store, but i seemed to go got it when it was in the Beta and for free, but now its 99¢, and with the App, you just drag a image into it, and give it a size Vertical or Horizontal.
FYI, if you need to resize a lot of images, you can also use Automator, and make a Workflow. I have 1 for making images 800x600 and i can do this in batches 100+ images at a time, or just the one if need be. Something to think about also if you are continuously having to resize images too.
Also, if you are wanting to resize to use in a forum like this, you can use a Image Hosting site like
imgur.com where you can set your image size, and a folder you want it to go into, (within your account) and you drag a image into the Safari window and it will rezise the image, then give you a bunch of URL's that you can use to post straight into say Mac-Forums post, using the
tags. This way i like because it leaves me with the original size image on my Mac, and then different sizes in my Imgur account. . .
Just another couple of ways to do it, as you didn't really give us too much info as to why or where you are putting the resized image
Cheers