Whenever an advertiser sends me an advert banner that is bigger than the required size, I use the site to make the image smaller. The site can be helpful too whenever you want to make picture smaller size before uploading it to Facebook, Twitter etc.
How To Resize Image Online
==> Go to : http://www.imageresize.org
==> If the image you want to resize is on your computer, click on "browse" to select it and Click "upload" when you are done.
==> If the image is somewhere on the web, click the "From Web" tab, enter the URL of the image and click "Fetch"
Once the image is loaded onto the dashboard, you will see the options to crop or resize the image. Now, let's assume you want to resize the image to 300x250.
==> From the "Change Resize Image To" dropdown box, select "Custom Size"
==> Untick "Preserve Aspect ratio"
==> Change the width to 300 pixels
==> Change the height to 250 pixels
==> Select your desired image format
==> Click "Process Image"
You will be redirected to a page displaying your resized image.
Right-click on the image and select Save image as to save or use the Fb share button to share it on Facebook.
I hope it helps.